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      HDF5 File Format Specification Version 1.1
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    <center>
    <table border=0 width=90%>
    <tr>
    <td valign=top>
    <ol type="I">
      <li><a href="#Intro">Introduction</a>
      <li><a href="#FileMetaData">Disk Format Level 0 - File Metadata</a>
        <font size=-2>
	<ol type="A">
	  <li><a href="#SuperBlock">Disk Format Level 0A - File Signature and Super Block</a>
	  <li><a href="#DriverInfo">Disk Format Level 0B - File Driver Info</a>
	</ol>
        </font>
      <li><a href="#FileInfra">Disk Format Level 1 - File Infrastructure</a>
        <font size=-2>
	<ol type="A">
	  <li><a href="#Btrees">Disk Format Level 1A - B-link Trees and B-tree Nodes</a>
	  <li><a href="#SymbolTable">Disk Format Level 1B - Group</a>
	  <li><a href="#SymbolTableEntry">Disk Format Level 1C - Group Entry</a>
	  <li><a href="#LocalHeap">Disk Format Level 1D - Local Heaps</a>
	  <li><a href="#GlobalHeap">Disk Format Level 1E - Global Heap</a>
	  <li><a href="#FreeSpaceIndex">Disk Format Level 1F - Free-space Index</a>
	</ol>
        </font>
      <li><a href="#DataObject">Disk Format Level 2 - Data Objects</a>
        <font size=-2>
	<ol type="A">
	  <li><a href="#ObjectHeader">Disk Format Level 2a - Data Object Headers</a>
	    <ol type="1">
	      <li><a href="#NILMessage">Name: NIL</a>                                                       <!-- 0x0000 -->
	      <li><a href="#SimpleDataSpace">Name: Simple Dataspace</a>                                     <!-- 0x0001 -->
<!--          <li><a href="#DataSpaceMessage">Name: Complex Dataspace</a>  -->                              <!-- 0x0002 -->
	      <li><a href="#ReservedMessage_0002">Name: Reserved - not assigned yet</a>                     <!-- 0x0002 -->
	      <li><a href="#DataTypeMessage">Name: Datatype</a>                                             <!-- 0x0003 -->
	      <li><a href="#OldFillValueMessage">Name: Data Storage - Fill Value (Old)</a>                  <!-- 0x0004 -->
	      <li><a href="#FillValueMessage">Name: Data Storage - Fill Value</a>                           <!-- 0x0005 -->
	    </ol>
        </ol>
        </font>
    </ol>
    </td><td>&nbsp;&nbsp;</td><td valign=top>
    <ol type="I" start="4">

      <li><a href="#DataObject">Disk Format Level 2 - Data Objects</a>
        <font size=-2><i>(Continued)</i>
	<ol type=A>
	  <li><a href="#ObjectHeader">Disk Format Level 2a - Data Object Headers</a><i>(Continued)</i>
	    <ol type="1" start="7">
<!--          <li><a href="#CompactDataStorageMessage">Name: Data Storage - Compact</a>  -->                <!-- 0x0006 -->
	      <li><a href="#ReservedMessage_0006">Name: Reserved - not assigned yet</a>                     <!-- 0x0006 -->
	      <li><a href="#ExternalFileListMessage">Name: Data Storage - External Data Files</a>           <!-- 0x0007 -->
	      <li><a href="#LayoutMessage">Name: Data Storage - Layout</a>                                  <!-- 0x0008 -->
	      <li><a href="#ReservedMessage_0009">Name: Reserved - not assigned yet</a>                     <!-- 0x0009 -->
	      <li><a href="#ReservedMessage_000A">Name: Reserved - not assigned yet</a>                     <!-- 0x000a -->
	      <li><a href="#FilterMessage">Name: Data Storage - Filter Pipeline</a>                	    <!-- 0x000b -->
	      <li><a href="#AttributeMessage">Name: Attribute</a>                                           <!-- 0x000c -->
	      <li><a href="#CommentMessage">Name: Object Comment</a>                                              <!-- 0x000d -->
	      <li><a href="#OldModifiedMessage">Name: Object Modification Date and Time (Old)</a>           <!-- 0x000e -->
              <li><a href="#SharedMessage">Name: Shared Object Message</a>                                  <!-- 0x000f -->
	      <li><a href="#ContinuationMessage">Name: Object Header Continuation</a>                       <!-- 0x0010 -->
	      <li><a href="#SymbolTableMessage">Name: Group Message</a>                                     <!-- 0x0011 -->
	      <li><a href="#ModifiedMessage">Name: Object Modification Date and Time</a>                    <!-- 0x0012 -->
	    </ol>
	  <li><a href="#DataStorage">Disk Format: Level 2b - Data Object Data Storage</a>
	</ol>
        </font>
      <LI><A href="#Appendix">Appendix</A>
    </ol>
</td></tr>
</table>
</center>

    <BR>
    <HR>


    <h2>Introduction</h2>

    <table align=right width=100>
    <tr><td>&nbsp;</td><td align=center>
        <hr>
        <img src="FF-IH_FileGroup.gif" alt="HDF5 Groups" hspace=15 vspace=15>
    </td><td>&nbsp;</td></tr>
    <tr><td>&nbsp;</td><td align=center>
        <strong>Figure 1:</strong> Relationships among the HDF5 root group, other groups, and objects
        <hr>
    </td><td>&nbsp;</td></tr>

    <tr><td>&nbsp;</td><td align=center>
        <img src="FF-IH_FileObject.gif" alt="HDF5 Objects" hspace=15 vspace=15>
    </td><td>&nbsp;</td></tr>
    <tr><td>&nbsp;</td><td align=center>
        <strong>Figure 2:</strong> HDF5 objects -- datasets, datatypes, or dataspaces
        <hr>
    </td><td>&nbsp;</td></tr>
    </table>


    <P>The format of an HDF5 file on disk encompasses several
      key ideas of the HDF4 and AIO file formats as well as
      addressing some shortcomings therein.  The new format is
      more self-describing than the HDF4 format and is more
      uniformly applied to data objects in the file.

    <P>An HDF5 file appears to the user as a directed graph.
      The nodes of this graph are the higher-level HDF5 objects
      that are exposed by the HDF5 APIs:

      <ul>
         <li>Groups
         <li>Datasets
         <li>Named datatypes
      </ul>

    <P>At the lowest level, as information is actually written to the disk,
       an HDF5 file is made up of the following objects:
      <ul>
         <li>A super block
         <li>B-tree nodes (containing either symbol nodes or raw data chunks)
         <li>Object headers
         <li>A global heap
         <li>Local heaps
         <li>Free space
      </ul>

    <P>The HDF5 library uses these low-level objects to represent the
      higher-level objects that are then presented to the user or
      to applications through the APIs.
      For instance, a group is an object header that contains a message that
      points to a local heap and to a B-tree which points to symbol nodes.
      A dataset is an object header that contains messages that describe
      datatype, space, layout, filters, external files, fill value, etc
      with the layout message pointing to either a raw data chunk or to a
      B-tree that points to raw data chunks.


    <h3>This Document</h3>

    <p>This document describes the lower-level data objects;
      the higher-level objects and their properties are described
      in the <a href="H5.user.html"><cite>HDF5 User Guide</cite></a>.

    <P>Three levels of information comprise the file format.
      Level 0 contains basic information for identifying and
      defining information about the file.  Level 1 information contains
      the information about the pieces of a file shared by many objects
      in the file (such as a B-trees and heaps).  Level 2 is the rest
      of the file and contains all of the data objects, with each object
      partitioned into header information, also known as
      <em>metadata</em>, and data.

    <p>The sizes of various fields in the following layout tables are
      determined by looking at the number of columns the field spans
      in the table.  There are three exceptions: (1) The size may be
      overridden by specifying a size in parentheses, (2) the size of
      addresses is determined by the <em>Size of Offsets</em> field
      in the super block and is indicated in this document with a
      superscripted 'O', and (3) the size of length fields is determined
      by the <em>Size of Lengths</em> field in the super block and is
      indicated in this document with a superscripted 'L'.

    <P>Values for all fields in this document should be treated as unsigned
      integers, unless otherwise noted in the description of a field.
      Additionally, all metadata fields are stored in little-endian byte
      order.
    </P>

    <BR>
    <HR>

    <h2><a name="FileMetaData">
	Disk Format: Level 0 - File Metadata</a></h2>

    <H3><A name="SuperBlock">
	Disk Format: Level 0A - File Signature and Super Block</A></H3>

    <P>The super block may begin at certain predefined offsets within
      the HDF5 file, allowing a block of unspecified content for
      users to place additional information at the beginning (and
      end) of the HDF5 file without limiting the HDF5 library's
      ability to manage the objects within the file itself.  This
      feature was designed to accommodate wrapping an HDF5 file in
      another file format or adding descriptive information to the
      file without requiring the modification of the actual file's
      information.  The super block is located by searching for the
      HDF5 file signature at byte offset 0, byte offset 512 and at
      successive locations in the file, each a multiple of two of
      the previous location, i.e.  0, 512, 1024, 2048, etc.

    <P>The super block is composed of a file signature, followed by
      super block and group version numbers, information
      about the sizes of offset and length values used to describe
      items within the file, the size of each group page,
      and a group entry for the root object in the file.

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  HDF5 Super Block Layout
	</caption>

	<tr>
	  <th>byte</th>
	  <th>byte</th>
	  <th>byte</th>
	  <th>byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>HDF5 File Signature (8 bytes)<br><br></td>
	</tr>

	<tr>
	  <td>Version # of Super Block</td>
	  <td>Version # of Global Free-space Storage</td>
	  <td>Version # of Root Group Symbol Table Entry</td>
	  <td>Reserved (zero)</td>
	</tr>

	<tr>
	  <td>Version # of Shared Header Message Format</td>
	  <td>Size of Offsets</td>
	  <td>Size of Lengths</td>
	  <td>Reserved (zero)</td>
	</tr>

	<tr>
	  <td colspan=2>Group Leaf Node K</td>
	  <td colspan=2>Group Internal Node K</td>
	</tr>

	<tr>
	  <td colspan=4>File Consistency Flags</td>
	</tr>

	<tr>
	  <td colspan=2 style="border:dotted;">Indexed Storage Internal Node K<sup>1</sup></td>
	  <td colspan=2 style="border:dotted;">Reserved (zero)<sup>1</sup></td>
	</tr>

	<tr>
	  <td colspan=4>Base Address<sup>O</sup></td>
	</tr>

	<tr>
	  <td colspan=4>Address of Global Free-space Heap<sup>O</sup></td>
	</tr>

	<tr>
	  <td colspan=4>End of File Address<sup>O</sup></td>
	</tr>

	<tr>
	  <td colspan=4>Driver Information Block Address<sup>O</sup></td>
	</tr>

	<tr>
	  <td colspan=4>Root Group Symbol Table Entry</td>
	</tr>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'O' the above table are
            <br>
            of the size specified in "Size of Offsets.")
        </td></tr>
        <tr><td>
            (Items marked with an '1' the above table are
            <br>
            new in version 1 of the superblock)
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>HDF5 File Signature</td>
	  <td>
            <P>This field contains a constant value and can be used to
	    quickly identify a file as being an HDF5 file.  The
	    constant value is designed to allow easy identification of
	    an HDF5 file and to allow certain types of data corruption
	    to be detected.  The file signature of an HDF5 file always
	    contains the following values:
            </P>

            <center>
	      <table border align=center cellpadding=4>
		<tr align=center>
		  <td align=right>Decimal:</td>
                  <td width="8%">137</td>
		  <td width="8%">72</td>
		  <td width="8%">68</td>
		  <td width="8%">70</td>
		  <td width="8%">13</td>
		  <td width="8%">10</td>
		  <td width="8%">26</td>
		  <td width="8%">10</td>
		</tr>

		<tr align=center>
		  <td align=right>Hexadecimal:</td>
		  <td>89</td>
		  <td>48</td>
		  <td>44</td>
		  <td>46</td>
		  <td>0d</td>
		  <td>0a</td>
		  <td>1a</td>
		  <td>0a</td>
		</tr>

		<tr align=center>
		  <td align=right>ASCII C Notation:</td>
		  <td>\211</td>
		  <td>H</td>
		  <td>D</td>
		  <td>F</td>
		  <td>\r</td>
		  <td>\n</td>
		  <td>\032</td>
		  <td>\n</td>
		</tr>
	      </table>
	    </center>
	    <br>

	    <P>This signature both identifies the file as an HDF5 file
	    and provides for immediate detection of common
	    file-transfer problems. The first two bytes distinguish
	    HDF5 files on systems that expect the first two bytes to
	    identify the file type uniquely. The first byte is
	    chosen as a non-ASCII value to reduce the probability
	    that a text file may be misrecognized as an HDF5 file;
	    also, it catches bad file transfers that clear bit
	    7. Bytes two through four name the format. The CR-LF
	    sequence catches bad file transfers that alter newline
	    sequences. The control-Z character stops file display
	    under MS-DOS. The final line feed checks for the inverse
	    of the CR-LF translation problem.  (This is a direct
	    descendent of the <A href="http://www.libpng.org/pub/png/spec/PNG-Rationale.html#R.PNG-file-signature">PNG</A> file
            signature.)
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
            </td>
	</tr>

	<tr>
	  <td>Version Number of the Super Block</td>
	  <td>
            <P>This value is used to determine the format of the
	    information in the super block.  When the format of the
	    information in the super block is changed, the version number
	    is incremented to the next integer and can be used to
	    determine how the information in the super block is
	    formatted.
            </P>

            <P>Values of 0 and 1 are defined for this field.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Version Number of the File Free-space Information</td>
	  <td>
            <P>This value is used to determine the format of the
	    information in the File Free-space Information.
            </P>
            <P>The only value currently valid in this field is '0', which
            indicates that the free space index is formatted as described
            <A href="#FreeSpaceIndex">below</A>.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Version Number of the Root Group Symbol Table Entry</td>
	  <td>
            <P>This value is used to determine the format of the
	    information in the Root Group Symbol Table Entry.  When the
            format of the information in that field is changed, the
	    version number is incremented to the next integer and can be
	    used to determine how the information in the field
	    is formatted.
            </P>
            <P>The only value currently valid in this field is '0', which
            indicates that the root group symbol table entry is formatted as
            described <A href="#SymbolTableEntry">below</A>.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Version Number of the Shared Header Message Format</td>
	  <td>
            <P>This value is used to determine the format of the
	    information in a shared object header message. Since the format
	    of the shared header messages differs from the other private
	    header messages, a version number is used to identify changes
	    in the format.
            </P>
            <P>The only value currently valid in this field is '0', which
            indicates that shared header messages are formatted as
            described <A href="#SharedMessage">below</A>.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Size of Offsets</td>
	  <td>
            <P>This value contains the number of bytes used to store
	    addresses in the file.  The values for the addresses of
	    objects in the file are offsets relative to a base address,
	    usually the address of the super block signature.  This
	    allows a wrapper to be added after the file is created
	    without invalidating the internal offset locations.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Size of Lengths</td>
	  <td>
            <P>This value contains the number of bytes used to store
	    the size of an object.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Group Leaf Node K</td>
	  <td>
            <P>Each leaf node of a group B-tree will have at
	    least this many entries but not more than twice this
	    many.  If a group has a single leaf node then it
	    may have fewer entries.
            </P>
            <P>This value must be greater than zero.
            </P>
            <P>See the <A href="#Btrees">description</A> of B-trees below.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Group Internal Node K</td>
	  <td>
            <P>Each internal node of a group B-tree will have at
	    least this many entries but not more than twice this
	    many.  If the group has only one internal
	    node then it might have fewer entries.
            </P>
            <P>This value must be greater than zero.
            </P>
            <P>See the <A href="#Btrees">description</A> of B-trees below.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
	</tr>

        <tr>
          <td>File Consistency Flags</td>
          <td>
            <P>This value contains flags to indicate information
            about the consistency of the information contained
            within the file.  Currently, the following bit flags are
            defined:
            <ul>
            <li>Bit 0 set indicates that the file is opened for
            write-access.
            <li>Bit 1 set indicates that the file has
            been verified for consistency and is guaranteed to be
            consistent with the format defined in this document.
            <li>Bits 2-31 are reserved for future use.
            </ul>
            Bit 0 should be
            set as the first action when a file is opened for write
            access and should be cleared only as the final action
            when closing a file.  Bit 1 should be cleared during
            normal access to a file and only set after the file's
            consistency is guaranteed by the library or a
            consistency utility.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
        </tr>

	<tr>
	  <td>Indexed Storage Internal Node K</td>
	  <td>
            <P>Each internal node of a indexed storage B-tree will have at
	    least this many entries but not more than twice this
	    many.  If the group has only one internal
	    node then it might have fewer entries.
            </P>
            <P>This value must be greater than zero.
            </P>
            <P>See the <A href="#Btrees">description</A> of B-trees below.
            </P>

            <P><EM>This field is present in version 1+ of the superblock.</EM>
            </P>
          </td>
	</tr>

        <tr>
          <td>Base Address</td>
          <td>
            <P>This is the absolute file address of the first byte of
            the HDF5 data within the file.  The library currently
            constrains this value to be the absolute file address
            of the super block itself when creating new files;
            future versions of the library may provide greater
            flexibility.  When opening an existing file and this address does
            not match the offset of the superblock, the library assumes
            that the entire contents of the HDF5 file have been adjusted in
            the file and adjusts the base address and end of file address to
            reflect their new positions in the file.  Unless otherwise noted,
            all other file addresses are relative to this base
            address.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
        </tr>

        <tr>
          <td>Address of Global Free-space Index</td>
          <td>
            <P>Free-space management is not yet defined in the HDF5
            file format and is not handled by the library.
            Currently this field always contains the
            <A href="#UndefinedAddress">undefined address</A>.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
        </tr>

        <tr>
          <td>End of File Address</td>
          <td>
            <P>This is the absolute file address of the first byte past
            the end of all HDF5 data.  It is used to determine whether a
            file has been accidentally truncated and as an address where
            file data allocation can occur if space from the free list is
            not used.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
        </tr>

        <tr>
          <td>Driver Information Block Address</td>
          <td>
            <P>This is the relative file address of the file driver
            information block which contains driver-specific
            information needed to reopen the file. If there is no
            driver information block then this entry should be the
            <A href="#UndefinedAddress">undefined address</A>.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
        </tr>

        <tr>
          <td>Root Group Symbol Table Entry</td>
          <td>
            <P>This is the <A href="#SymbolTableEntry">symbol table entry</A>
            of the root group, which serves as the entry point into
            the group graph for the file.
            </P>

            <P><EM>This field is present in version 0+ of the superblock.</EM>
            </P>
          </td>
        </tr>
      </table>
    </div>

    <H3><A name="DriverInfo">
	Disk Format: Level 0B - File Driver Info</A></H3>

    <p>The <em>file driver information block</em> is an optional region of the
      file which contains information needed by the file driver in
      order to reopen a file.  The format of the file driver information
      block is:

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Driver Information Block
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
	</tr>

        <tr>
          <td>Version</td>
          <td colspan=3>Reserved (zero)</td>
        </tr>

        <tr>
          <td colspan=4>Driver Information Size (4 bytes)</td>
        </tr>

        <tr>
          <td colspan=4><br>Driver Identification (8 bytes)<br><br></td>
        </tr>

        <tr>
          <td colspan=4><br><br>Driver Information (<em>n</em> bytes)<br><br><br></td>
        </tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Version</td>
          <td>
            <P>The version number of the driver information block. The
            file format documented here is version zero.
            </P>
          </td>
        </tr>

        <tr>
          <td>Driver Information Size</td>
          <td>
            <P>The size in bytes of the Driver Information part of this
            structure.
            </P>
          </td>
        </tr>

        <tr>
          <td>Driver Identification</td>
          <td>
            <P>This is an eight-byte ASCII string without null
            termination which identifies the driver and version number
            of the Driver Information block. The predefined drivers
            supplied with the HDF5 library are identified by the
            letters <code>NCSA</code> followed by the first four characters of
            the driver name. If the Driver Information block is not
            the original version then the last letter(s) of the
            identification will be replaced by a version number in
            ASCII.
            </P>
            <P>
            For example, the various versions of the <em>multi driver</em>
            will be identified by <code>NCSAmult</code>.
            (<code>NCSAmult</code> is simply <code>NCSAmulti</code> truncated
            to eight characters.  Subsequent identifiers will be created by
            substituting sequential numerical values for the final character,
            starting with zero.) <em>multi driver</em> is the only default driver that
            is encoded in this field.
            </P>
            <P>
            Identification for user-defined drivers
            is eight-byte long and arbitrary but should be unique and avoid
            the four character prefix "NCSA".
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Driver Information</td>
          <td>Driver information is encoded/decoded in a format defined by the
            file driver. <em>multi driver</em> is the only default driver that has driver
            information stored in this field.  Its format is explained in the
            following block.</td>
        </tr>
      </table>
    </div>

    <BR>
    <P><em>Multi driver</em> has the following format:</P>

    <div align=center>
      <table class=format>
	<caption>
	  Multi Driver Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Member Mapping</td>
	  <td>Member Mapping</td>
	  <td>Member Mapping</td>
	  <td>Member Mapping</td>
	</tr>

	<tr>
	  <td>Member Mapping</td>
	  <td>Member Mapping</td>
	  <td>Reserved</td>
	  <td>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4><br>Address of Member File 1<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>End of Address for Member File 1<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Address of Member File 2<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>End of Address for Member File 2<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>... ...<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Name of Member File 1<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Name of Member File 2<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>... ...<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
	  <th width="30%">Field Name</th>
	  <th>Description</th>
	</tr>

	<tr>
	  <td>Member Mapping</td>
	  <td><P><em>Multi driver</em> enables different types of HDF5 data and
            metadata to be written to separate files. These files are viewed by the
            library as a single virtual HDF5 file with a single file address.
            It allows maximal 6 files to be created.
	    In sequence, these <em>Member Mapping</em> fields are for super block,
            B-tree, raw data, global heap, local heap,
	    and object header.  More than one type of data can be written to the
            same file.</P>
	    <P>These <em>Member Mapping</em> fields are integer values from 1 to 6
            indicating how the data can be mapped to or merged with another type of
            data.
	        <table class=list>
                    <tr>
                    <th width="30%">Member Mapping</th>
                    <th align=left>Description</th>
                    </tr>
                    <tr>
                        <td align=center>1</td>
                        <td>The super block data.</td>
                    </tr>
                    <tr>
                        <td align=center>2</td>
                        <td>The B-tree data.</td>
                    </tr>
                    <tr>
                        <td align=center>3</td>
                        <td>The raw data.</td>
                    </tr>
                    <tr>
                        <td align=center>4</td>
                        <td>The global heap data.</td>
                    </tr>
                    <tr>
                        <td align=center>5</td>
                        <td>The local heap data.</td>
                    </tr>
                    <tr>
                        <td align=center>6</td>
                        <td>The object header data.</td>
                    </tr>
                </table></P>
            For example, if the third field has the value 3 and all the rest have the
            value 1, it means there are two files, one for raw data, one for super block,
            B-tree, global heap, local heap, and object header.
          </td>
	</tr>

	<tr>
	  <td>Reserved</td>
	  <td><P>These fields are reserved and should always be zero.</P></td>
	</tr>

	<tr>
	  <td>Address of Member File</td>
	  <td><P>Specifies the virtual address. A normally eight-byte integer with
            the value from <em>0</em> (zero) to maximal value,
	    at which the member file starts.</P></td>
	</tr>

	<tr>
	  <td>End of Address for Member File</td>
	  <td><P>The end of allocated address for the member file. A normally eight-byte
            integer value.</P></td>
	</tr>

	<tr>
	  <td>Name of Member File</td>
	  <td><P>The null-terminated name of member file.  Its length should be multiples of
	    8 bytes.  Additional bytes will be padded with <em>NULL</em>s. The default naming
            convention is <em>%%s-X.h5</em>, where <em>X</em> is one of the letters
            <em>s</em> (for super block), <em>b</em> (for B-tree), <em>r</em> (for raw data),
            <em>g</em> (for global heap), <em>l</em> (for local heap), and <em>o</em> (for
            object header). The name for the whole HDF5 file will substitute the <em>%s</em>
            in the string.
          </P>
          </td>
	</tr>
      </table>
    </div>

    <BR>
    <HR>

    <h2><a name="FileInfra">
        Disk Format: Level 1 - File Infrastructure</a></h2>
    <h3><a name="Btrees">Disk Format: Level 1A - B-link Trees and B-tree Nodes</a></h3>

    <p>B-link trees allow flexible storage for objects which tend to grow
      in ways that cause the object to be stored discontiguously.  B-trees
      are described in various algorithms books including "Introduction to
      Algorithms" by Thomas H. Cormen, Charles E. Leiserson, and Ronald
      L. Rivest.  The B-link tree, in which the sibling nodes at a
      particular level in the tree are stored in a doubly-linked list,
      is described in the "Efficient Locking for Concurrent Operations
      on B-trees" paper by Phillip Lehman and S. Bing Yao as published
      in the <cite>ACM Transactions on Database Systems</cite>, Vol. 6,
      No. 4, December 1981.

    <p>The B-link trees implemented by the file format contain one more
      key than the number of children.  In other words, each child
      pointer out of a B-tree node has a left key and a right key.
      The pointers out of internal nodes point to sub-trees while
      the pointers out of leaf nodes point to symbol nodes and
      raw data chunks.
      Aside from that difference, internal nodes and leaf nodes
      are identical.

    <br>
    <div align=center>
      <table class=format>
        <caption>
          B-tree Nodes
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>

        <tr>
          <td colspan=4>Signature</td>

        <tr>
          <td>Node Type</td>
          <td>Node Level</td>
          <td colspan=2>Entries Used</td>

        <tr>
          <td colspan=4>Address of Left Sibling<sup>O</sup></td>

        <tr>
          <td colspan=4>Address of Right Sibling<sup>O</sup></td>

        <tr>
          <td colspan=4>Key 0 (variable size)</td>

        <tr>
          <td colspan=4>Address of Child 0<sup>O</sup></td>

        <tr>
          <td colspan=4>Key 1 (variable size)</td>

        <tr>
          <td colspan=4>Address of Child 1<sup>O</sup></td>

        <tr>
          <td colspan=4>...</td>

        <tr>
          <td colspan=4>Key 2<em>K</em> (variable size)</td>

        <tr>
          <td colspan=4>Address of Child 2<em>K</em><sup>O</sup></td>

        <tr>
          <td colspan=4>Key 2<em>K</em>+1 (variable size)</td>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'O' the above table are
            <br>
            of the size specified in "Size of Offsets.")
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Signature</td>
          <td>
            <P>The ASCII character string "<code>TREE</code>" is
            used to indicate the
            beginning of a B-link tree node.  This gives file
            consistency checking utilities a better chance of
            reconstructing a damaged file.
            </P>
          </td>
        </tr>

        <tr>
          <td>Node Type</td>
          <td>
            <P>Each B-link tree points to a particular type of data.
            This field indicates the type of data as well as
            implying the maximum degree <em>K</em> of the tree and
            the size of each Key field.
            </P>

            <table class=list>
                <tr>
                <th width="30%">Node Type</th>
                <th align=left>Description</th>
                </tr>
                <tr>
                    <td align=center>0</td>
                    <td>This tree points to group nodes.</td>
                </tr>
                <tr>
                    <td align=center>1</td>
                    <td>This tree points to raw data chunk nodes.</td>
                </tr>
            </table>
          </td>
        </tr>

        <tr>
          <td>Node Level</td>
          <td>
            <P>The node level indicates the level at which this node
            appears in the tree (leaf nodes are at level zero).  Not
            only does the level indicate whether child pointers
            point to sub-trees or to data, but it can also be used
            to help file consistency checking utilities reconstruct
            damaged trees.
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Entries Used</td>
          <td>
            <P>This determines the number of children to which this
            node points.  All nodes of a particular type of tree
            have the same maximum degree, but most nodes will point
            to less than that number of children.  The valid child
            pointers and keys appear at the beginning of the node
            and the unused pointers and keys appear at the end of
            the node.  The unused pointers and keys have undefined
            values.
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Address of Left Sibling</td>
          <td>
            <P>This is the relative file address of the left sibling of
            the current node.  If the current
            node is the left-most node at this level then this field
            is the <A href="#UndefinedAddress">undefined address</A>.
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Address of Right Sibling</td>
          <td>
            <P>This is the relative file address of the right sibling of
            the current node.  If the current
            node is the right-most node at this level then this
            field is the <A href="#UndefinedAddress">undefined address</A>.
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Keys and Child Pointers</td>
          <td>
            <P>Each tree has 2<em>K</em>+1 keys with 2<em>K</em>
            child pointers interleaved between the keys.  The number
            of keys and child pointers actually containing valid
            values is determined by the node's <em>Entries Used</em> field.
            If that field is <em>N</em> then the B-link tree contains
            <em>N</em> child pointers and <em>N</em>+1 keys.
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Key</td>
          <td>
            <P>The format and size of the key values is determined by
            the type of data to which this tree points.  The keys are
            ordered and are boundaries for the contents of the child
            pointer; that is, the key values represented by child
            <em>N</em> fall between Key <em>N</em> and Key
            <em>N</em>+1. Whether the interval is open or closed on
            each end is determined by the type of data to which the
            tree points.
            </P>

            <P>
            The format of the key depends on the node type.
            For nodes of node type 0 (group nodes), the key is formatted as
            follows:
            <center>
            <table class=list>
            <tr>
              <td width=30%>A single field of <i>Size of Lengths</i>
                bytes:</td>
              <td>Indicates the byte offset into the local heap
                for the first object name in the subtree which
                that key describes.
              </td>
            </tr>
            </table>
            </center>
            </P>

            <P>
            For nodes of node type 1 (chunked raw data nodes), the key is
            formatted as follows:
            <center>
            <table class=list>
            <tr>
              <td width=30%>Bytes 1-4:</td>
              <td>Size of chunk in bytes.</td>
            </tr>
            <tr>
              <td>Bytes 4-8:</td>
              <td>Filter mask, a 32-bit bitfield indicating which
                filters have been skipped for this chunk.  Each filter
                has an index number in the pipeline (starting at 0, with
                the first filter to apply) and if that filter is skipped,
                the bit corresponding to it's index is set.</td>
            </tr>
            <tr>
              <td><em>N</em> 64-bit fields:</td>
              <td>A 64-bit index indicating the offset of the
                chunk within the dataset where <i>N</i> is the number
                of dimensions of the dataset.  For example, if
                a chunk in a 3-dimensional dataset begins at the
                position <code>[5,5,5]</code>, there will be three
                such 64-bit indices, each with the value of
                <code>5</code>.</td>
            </tr>
            </table>
            </center>
            </P>
          </td>
        </tr>

        <tr valign=top>
          <td>Child Pointer</td>
          <td>
            <P>The tree node contains file addresses of subtrees or
            data depending on the node level.  Nodes at Level 0 point
            to data addresses, either raw data chunk or group nodes.
            Nodes at non-zero levels point to other nodes of the
            same B-tree.
            </P>
            <P>For raw data chunk nodes, the child pointer is the address
            of a single raw data chunk.  For group nodes, the child pointer
            points to a <A href="#SymbolTable">symbol table</A>, which contains
            information for multiple symbol table entries.
            </P>
          </td>
        </tr>
      </table>
    </div>

    <p>
     Conceptually, each B-tree node looks like this:
     <center>
     <table>
       <tr valign=top align=center>
         <td>key[0]</td><td>&nbsp;</td>
         <td>child[0]</td><td>&nbsp;</td>
         <td>key[1]</td><td>&nbsp;</td>
         <td>child[1]</td><td>&nbsp;</td>
         <td>key[2]</td><td>&nbsp;</td>
         <td>...</td><td>&nbsp;</td>
         <td>...</td><td>&nbsp;</td>
         <td>key[<i>N</i>-1]</td><td>&nbsp;</td>
         <td>child[<i>N</i>-1]</td><td>&nbsp;</td>
         <td>key[<i>N</i>]</td>
       </tr>
     </table>
     </center>
     <br>

     where child[<i>i</i>] is a pointer to a sub-tree (at a level
     above Level 0) or to data (at Level 0).
     Each key[<i>i</i>] describes an <i>item</i> stored by the B-tree
     (a chunk or an object of a group node).  The range of values
     represented by child[<i>i</i>] is indicated by key[<i>i</i>]
     and key[<i>i</i>+1].


    <p>The following question must next be answered:
     "Is the value described by key[<i>i</i>] contained in
     child[<i>i</i>-1] or in child[<i>i</i>]?"
     The answer depends on the type of tree.
     In trees for groups (node type 0) the object described by
     key[<i>i</i>] is the greatest object contained in
     child[<i>i</i>-1] while in chunk trees (node type 1) the
     chunk described by key[<i>i</i>] is the least chunk in
     child[<i>i</i>].

    <p>That means that key[0] for group trees is sometimes unused;
     it points to offset zero in the heap, which is always the
     empty string and compares as "less-than" any valid object name.

    <p>And key[<i>N</i>] for chunk trees is sometimes unused;
     it contains a chunk offset which compares as "greater-than"
     any other chunk offset and has a chunk byte size of zero
     to indicate that it is not actually allocated.


    <h3><a name="SymbolTable">Disk Format: Level 1B - Group and Symbol Nodes</a></h3>

    <p>A group is an object internal to the file that allows
      arbitrary nesting of objects within the file (including other groups).
      A group maps a set of names in the group to a set of relative
      file addresses where objects with those names are located in
      the file.  Certain metadata for an object to which the group points
      can be cached in the group's symbol table in addition to the
      object's header.

    <p>An HDF5 object name space can be stored hierarchically by
      partitioning the name into components and storing each
      component in a group.  The group entry for a
      non-ultimate component points to the group containing
      the next component.  The group entry for the last
      component points to the object being named.

    <p>A group is a collection of group nodes pointed
      to by a B-link tree.  Each group node contains entries
      for one or more symbols.  If an attempt is made to add a
      symbol to an already full group node containing
      2<em>K</em> entries, then the node is split and one node
      contains <em>K</em> symbols and the other contains
      <em>K</em>+1 symbols.

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Group Node (A Leaf of a B-tree)
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>

        <tr>
          <td colspan=4>Signature</td>

        <tr>
          <td>Version Number</td>
          <td>Reserved (0)</td>
          <td colspan=2>Number of Symbols</td>

        <tr>
          <td colspan=4><br><br>Group Entries<br><br><br></td>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Signature</td>
          <td>
            <P>The ASCII character string "<code>SNOD</code>" is
            used to indicate the
            beginning of a group node.  This gives file
            consistency checking utilities a better chance of
            reconstructing a damaged file.
            </P>
          </td>
        </tr>

        <tr>
          <td>Version Number</td>
          <td>
            <P>The version number for the group node.  This
            document describes version 1. (There is no version '0'
            of the group node)
            </P>
          </td>
        </tr>

        <tr>
          <td>Number of Symbols</td>
          <td>
            <P>Although all group nodes have the same length,
            most contain fewer than the maximum possible number of
            symbol entries.  This field indicates how many entries
            contain valid data.  The valid entries are packed at the
            beginning of the group node while the remaining
            entries contain undefined values.
            </P>
          </td>
        </tr>

        <tr>
          <td>Group Entries</td>
          <td>
            <P>Each symbol has an entry in the group node.
            The format of the entry is described below.
            There are 2<EM>K</EM> entries in each group node, where
            <EM>K</EM> is the "Group Leaf Node K" value from the
            <A href="#SuperBlock">super block</A>.
            </P>
          </td>
        </tr>
      </table>
    </div>

    <h3><a name="SymbolTableEntry">
        Disk Format: Level 1C - Group Entry </a></h3>

    <p>Each group entry in a group node is designed
      to allow for very fast browsing of stored objects.
      Toward that design goal, the group entries
      include space for caching certain constant metadata from the
      object header.

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Group Entry
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
        </tr>

        <tr>
          <td colspan=4>Name Offset<sup>O</sup></td>
        </tr>

        <tr>
          <td colspan=4>Object Header Address<sup>O</sup></td>
        </tr>

        <tr>
          <td colspan=4>Cache Type</td>
        </tr>

        <tr>
          <td colspan=4>Reserved</td>
        </tr>

        <tr>
          <td colspan=4><br><br>Scratch-pad Space (16 bytes)<br><br><br></td>
        </tr>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'O' the above table are
            <br>
            of the size specified in "Size of Offsets.")
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Name Offset</td>
          <td>
            <P>This is the byte offset into the group local
            heap for the name of the object. The name is null
            terminated.
            </P>
          </td>
        </tr>

        <tr>
          <td>Object Header Address</td>
          <td>
            <P>Every object has an object header which serves as a
            permanent location for the object's metadata.  In addition
            to appearing in the object header, some metadata can be
            cached in the scratch-pad space.
            </P>
          </td>
        </tr>

        <tr>
          <td>Cache Type</td>
          <td>
            <P>The cache type is determined from the object header.
            It also determines the format for the scratch-pad space:
            <br>
            <table class=list>
                <tr align=left>
                    <th>Type:</th>
                    <th>Description:</th>
                </tr>
                <tr>
                    <td width="10%" align=center>0</td>
                    <td>No data is cached by the group entry.  This
                        is guaranteed to be the case when an object header
                        has a link count greater than one.
                    </td>
                </tr>
                <tr>
                    <td align=center>1</td>
                    <td>Object header metadata is cached in the group
                        entry.  This implies that the group
                        entry refers to another group.
                    </td>
                </tr>
                <tr>
                    <td align=center>2</td>
                    <td>The entry is a symbolic link.  The first four bytes
                        of the scratch-pad space are the offset into the local
                        heap for the link value.  The object header address
                        will be undefined.
                    </td>
                </tr>
                <tr>
                    <td align=center><em>N</em></td>
                    <td>Other cache values can be defined later and
                        libraries that do not understand the new values will
                        still work properly.
                    </td>
                </tr>
            </table>
            </P>
          </td>
        </tr>

        <tr>
          <td>Reserved</td>
          <td>
            <P>These four bytes are present so that the scratch-pad
            space is aligned on an eight-byte boundary.  They are
            always set to zero.
            </P>
          </td>
        </tr>

        <tr>
          <td>Scratch-pad Space</td>
          <td>
            <P>This space is used for different purposes, depending
            on the value of the Cache Type field. Any metadata
            about a dataset object represented in the scratch-pad
            space is duplicated in the object header for that
            dataset.  This metadata can include the datatype
            and the size of the dataspace for a dataset whose datatype
            is atomic and whose dataspace is fixed and less than
            four dimensions.
            </P>
            <P>
            Furthermore, no data is cached in the group
            entry scratch-pad space if the object header for
            the group entry has a link count greater than
            one.
            </P>
          </td>
        </tr>
      </table>
    </div>

    <h4>Format of the Scratch-pad Space</h4>

    <p>The group entry scratch-pad space is formatted
      according to the value in the Cache Type field.

    <p>If the Cache Type field contains the value zero
      <code>(0)</code> then no information is
      stored in the scratch-pad space.

    <p>If the Cache Type field contains the value one
      <code>(1)</code>, then the scratch-pad space
      contains cached metadata for another object header
      in the following format:

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Object Header Scratch-pad Format
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>

        <tr>
          <td colspan=4>Address of B-tree<sup>O</sup></td>

        <tr>
          <td colspan=4>Address of Name Heap<sup>O</sup></td>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'O' the above table are
            <br>
            of the size specified in "Size of Offsets.")
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Address of B-tree</td>
          <td>
            <P>This is the file address for the root of the
            group's B-tree.
            </P>
          </td>
        </tr>

        <tr>
          <td>Address of Name Heap</td>
          <td>
            <P>This is the file address for the group's local
            heap, in which are stored the group's symbol names.
            </P>
          </td>
        </tr>
      </table>
    </div>


    <P>If the Cache Type field contains the value two
      <code>(2)</code>, then the scratch-pad space
      contains cached metadata for another symbolic link
      in the following format:

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Symbolic Link Scratch-pad Format
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
        </tr>

        <tr>
          <td colspan=4>Offset to Link Value</td>
        </tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Offset to Link Value</td>
          <td>
            <P>The value of a symbolic link (that is, the name of the
            thing to which it points) is stored in the local heap.
            This field is the 4-byte offset into the local heap for
            the start of the link value, which is null terminated.
            </P>
          </td>
        </tr>
      </table>
    </div>

    <h3><a name="LocalHeap">Disk Format: Level 1D - Local Heaps</a></h3>

    <P>A heap is a collection of small heap objects.  Objects can be
      inserted and removed from the heap at any time.
      The address of a heap does not change once the heap is created.
      References to objects are stored in the group table;
      the names of those objects are stored in the local heap.
    </P>

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Local Heap
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
        </tr>

        <tr>
          <td colspan=4>Signature</td>
        </tr>

        <tr>
          <td>Version</td>
          <td colspan=3>Reserved (zero)</td>
        </tr>

        <tr>
          <td colspan=4>Data Segment Size<sup>L</sup></td>
        </tr>

        <tr>
          <td colspan=4>Offset to Head of Free-list<sup>L</sup></td>
        </tr>

        <tr>
          <td colspan=4>Address of Data Segment<sup>O</sup></td>
        </tr>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'L' the above table are
            <br>
            of the size specified in "Size of Lengths.")
        </td></tr>
        <tr><td>
            (Items marked with an 'O' the above table are
            <br>
            of the size specified in "Size of Offsets.")
        </td></tr>
      </table>
    </div>

    <p>
    <center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Signature</td>
          <td>
            <P>The ASCII character string "<code>HEAP</code>"
            is used to indicate the
            beginning of a heap.  This gives file consistency
            checking utilities a better chance of reconstructing a
            damaged file.
            </P>
          </td>
        </tr>

        <tr>
          <td>Version</td>
          <td>
            <P>Each local heap has its own version number so that new
            heaps can be added to old files.  This document
            describes version zero (0) of the local heap.
            </P>
          </td>
        </tr>

        <tr>
          <td>Data Segment Size</td>
          <td>
            <P>The total amount of disk memory allocated for the heap
            data.  This may be larger than the amount of space
            required by the objects stored in the heap.  The extra
            unused space in the heap holds a linked list of free blocks.
            </P>
          </td>
        </tr>

        <tr>
          <td>Offset to Head of Free-list</td>
          <td>
            <P>This is the offset within the heap data segment of the
            first free block (or the
            <A href="#UndefinedAddress">undefined address</A> if there is no
            free block).  The free block contains "Size of Lengths" bytes that
            are the offset of the next free block (or the
            value '1' if this is the
            last free block) followed by "Size of Lengths" bytes that store
            the size of this free block.  The size of the free block includes
            the space used to store the offset of the next free block and
            the of the current block, making the minimum size of a free block
            2 * "Size of Lengths".
            </P>
          </td>
        </tr>

        <tr>
          <td>Address of Data Segment</td>
          <td>
            <P>The data segment originally starts immediately after
            the heap header, but if the data segment must grow as a
            result of adding more objects, then the data segment may
            be relocated, in its entirety, to another part of the
            file.
            </P>
          </td>
        </tr>
      </table>
    </center>

    <p>Objects within the heap should be aligned on an 8-byte boundary.

    <h3><a name="GlobalHeap">Disk Format: Level 1E - Global Heap</a></h3>

    <P>Each HDF5 file has a global heap which stores various types of
      information which is typically shared between datasets.  The
      global heap was designed to satisfy these goals:

    <ol type="A">
      <li>Repeated access to a heap object must be efficient without
        resulting in repeated file I/O requests. Since global heap
        objects will typically be shared among several datasets, it is
        probable that the object will be accessed repeatedly.
      <li>Collections of related global heap objects should result in
        fewer and larger I/O requests.  For instance, a dataset of
        object references will have a global heap object for each
        reference.  Reading the entire set of object references
        should result in a few large I/O requests instead of one small
        I/O request for each reference.
      <li>It should be possible to remove objects from the global heap
        and the resulting file hole should be eligible to be reclaimed
        for other uses.
    </ol>
    </P>

    <P>The implementation of the heap makes use of the memory
      management already available at the file level and combines that
      with a new top-level object called a <em>collection</em> to
      achieve Goal B. The global heap is the set of all collections.
      Each global heap object belongs to exactly one collection and
      each collection contains one or more global heap objects. For
      the purposes of disk I/O and caching, a collection is treated as
      an atomic object.
    </P>

    <P>The HDF5 library creates global heap collections as needed, so there may
        be multiple collections throughout the file. The set of all of them is
        abstractly called the "global heap", although they don't actually link
        to each other, and there is no global place in the file where you can
        discover all of the collections.  The collections are found simply by
        finding a reference to one through another object in the file (eg.
        variable-length datatype elements, etc).
    </P>

    <br>
    <div align=center>
      <table class=format>
        <caption>
          A Global Heap Collection
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
        </tr>

        <tr>
          <td colspan=4>Signature</td>
        </tr>

        <tr>
          <td>Version</td>
          <td colspan=3>Reserved (zero)</td>
        </tr>

        <tr>
          <td colspan=4>Collection Size<sup>L</sup></td>
        </tr>

        <tr>
          <td colspan=4><br>Global Heap Object 1<br><br></td>
        </tr>

        <tr>
          <td colspan=4><br>Global Heap Object 2<br><br></td>
        </tr>

        <tr>
          <td colspan=4><br>...<br><br></td>
        </tr>

        <tr>
          <td colspan=4><br>Global Heap Object <em>N</em><br><br></td>
        </tr>

        <tr>
          <td colspan=4><br>Global Heap Object 0 (free space)<br><br></td>
        </tr>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'L' the above table are
            <br>
            of the size specified in "Size of Lengths.")
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Signature</td>
          <td>
            <P>The ASCII character string "<code>GCOL</code>"
            is used to indicate the
            beginning of a collection.  This gives file consistency
            checking utilities a better chance of reconstructing a
            damaged file.
            </P>
          </td>
        </tr>

        <tr>
          <td>Version</td>
          <td>
            <P>Each collection has its own version number so that new
            collections can be added to old files.  This document
            describes version one (1) of the collections (there is no
            version zero (0)).
            </P>
          </td>
        </tr>

        <tr>
          <td>Collection Size</td>
          <td>
            <P>This is the size in bytes of the entire collection
            including this field.  The default (and minimum)
            collection size is 4096 bytes which is a typical file
            system block size.  This allows for 127 16-byte heap
            objects plus their overhead (the collection header of 16 bytes
            and the 16 bytes of information about each heap object).
            </P>
          </td>
        </tr>

        <tr>
          <td>Global Heap Object 1 through <em>N</em></td>
          <td>
            <P>The objects are stored in any order with no
            intervening unused space.
            </P>
          </td>
        </tr>

        <tr>
          <td>Global Heap Object 0</td>
          <td>
            <P>Global Heap Object 0 (zero), when present, represents the free
            space in the collection.  Free space always appears at the end of
            the collection.  If the free space is too small to store the header
            for Object 0 (described below) then the header is implied and the
            collection contains no free space.
            </P>
          </td>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Global Heap Object
        </caption>

        <tr>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
          <th>byte</th>
        </tr>

        <tr>
          <td colspan=2>Heap Object ID</td>
          <td colspan=2>Reference Count</td>
        </tr>

        <tr>
          <td colspan=4>Reserved</td>
        </tr>

        <tr>
          <td colspan=4>Object Size<sup>L</sup></td>
        </tr>

        <tr>
          <td colspan=4><br>Object Data<br><br></td>
        </tr>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'L' the above table are
            <br>
            of the size specified in "Size of Lengths.")
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Heap Object ID</td>
          <td>
            <P>Each object has a unique identification number within a
            collection.  The identification numbers are chosen so that
            new objects have the smallest value possible with the
            exception that the identifier <code>0</code> always refers to the
            object which represents all free space within the
            collection.
            </P>
          </td>
        </tr>

        <tr>
          <td>Reference Count</td>
          <td>
            <P>All heap objects have a reference count field.  An
            object which is referenced from some other part of the
            file will have a positive reference count. The reference
            count for Object 0 is always zero.
            </P>
          </td>
        </tr>

        <tr>
          <td>Reserved</td>
          <td>
            <P>Zero padding to align next field on an 8-byte boundary.
            </P>
          </td>
        </tr>

        <tr>
          <td>Object Size</td>
          <td>
            <P>This is the size of the object data stored for the object.
            The actual storage space allocated for the object data is rounded
            up to a multiple of eight.
            </P>
          </td>
        </tr>

        <tr>
          <td>Object Data</td>
          <td>
            <P>The object data is treated as a one-dimensional array
            of bytes to be interpreted by the caller.
            </P>
          </td>
        </tr>
      </table>
    </div>

    <h3><a name="FreeSpaceIndex">Disk Format: Level 1F - Free-space Index</a></h3>

    <p>The free-space index is a collection of blocks of data,
      dispersed throughout the file, which are currently not used by
      any file objects.

    <p>The super block contains a pointer to root of the free-space description;
      that pointer is currently required to be the
      <A href="#UndefinedAddress">undefined address</A>.

    <p>The format of the free-space index is not defined at this time.

<!--
    <p>The Free-space Index is a collection of blocks of data,
      dispersed throughout the file, which are currently not used by
      any file objects.  The blocks of data are indexed by a B-tree of
      their length within the file.


    <p>Each B-tree page is composed of the following entries and
      B-tree management information, organized as follows:

    <p>
    <center>
      <table border cellpadding=4 width="80%">
        <caption align=bottom>
          <B>HDF5 Free-space Heap Page</B>
        </caption>

        <tr align=center>
          <th width="25%">byte</th>
          <th width="25%">byte</th>
          <th width="25%">byte</th>
          <th width="25%">byte</th>

        <tr align=center>
          <td colspan=4>Signature</td>
        <tr align=center>
          <td colspan=4>B-tree Left-link Offset</td>
        <tr align=center>
          <td colspan=4><br>Length of Free-block #1<br> <br></td>
        <tr align=center>
          <td colspan=4><br>Offset of Free-block #1<br> <br></td>
        <tr align=center>
          <td colspan=4>.<br>.<br>.<br></td>
        <tr align=center>
          <td colspan=4><br>Length of Free-block #n<br> <br></td>
        <tr align=center>
          <td colspan=4><br>Offset of Free-block #n<br> <br></td>
        <tr align=center>
          <td colspan=4>"High" Offset</td>
        <tr align=center>
          <td colspan=4>Right-link Offset</td>
      </table>
    </center>

    <p>
    <dl>
      <dt> The elements of the free-space heap page are described below:
      <dd>
        <dl>
          <dt>Signature: (4 bytes)
          <dd>The ASCII character string <code>FREE</code>
            is used to indicate the
            beginning of a free-space heap B-tree page.  This gives
            file consistency checking utilities a better chance of
            reconstructing a damaged file.

          <dt>B-tree Left-link Offset: (&lt;offset&gt; bytes)
          <dd>This value is used to indicate the offset of all offsets
            in the B-link-tree which are smaller than the value of the
            offset in entry #1.  This value is also used to indicate a
            leaf node in the B-link-tree by being set to all ones.

          <dt>Length of Free-block #n: (&lt;length&gt; bytes)
          <dd>This value indicates the length of an unused block in
            the file.

          <dt>Offset of Free-block #n: (&lt;offset&gt; bytes)
          <dd>This value indicates the offset in the file of an
            unused block in the file.

          <dt>"High" Offset: (4-bytes)
          <dd>This offset is used as the upper bound on offsets
            contained within a page when the page has been split.

          <dt>Right-link Offset: (&lt;offset&gt; bytes)
          <dd>This value is used to indicate the offset of the next
            child to the right of the parent of this group
            page.  When there is no node to the right, this value is
            all zeros.
        </dl>
    </dl>

    <p>The algorithms for searching and inserting objects in the
      B-tree pages are described fully in the Lehman and Yao paper,
      which should be read to provide a full description of the
      B-tree's usage.
-->

    <BR>
    <HR>

    <h2><a name="DataObject">Disk Format: Level 2 - Data Objects </a></h2>

    <P>Data objects contain the real information in the file.  These
      objects compose the scientific data and other information which
      are generally thought of as "data" by the end-user.  All the
      other information in the file is provided as a framework for
      these data objects.
    </P>

    <P>A data object is composed of header information and data
      information.  The header information contains the information
      needed to interpret the data information for the data object as
      well as additional "metadata" or pointers to additional
      "metadata" used to describe or annotate each data object.
    </P>

    <h3><a name="ObjectHeader">
	Disk Format: Level 2A - Data Object Headers</a></h3>

    <P>The header information of an object is designed to encompass
      all the information about an object, except for the data itself.
      This information includes
      the dataspace, datatype, information about how the data
      is stored on disk (in external files, compressed, broken up in
      blocks, etc.), as well as other information used by the library
      to speed up access to the data objects or maintain a file's
      integrity.  Information stored by user applications as attributes
      is also stored in the object's header.  The header of each object is
      not necessarily located immediately prior to the object's data in the
      file and in fact may be located in any position in the file.  The order
      of the messages in an object header is not significant.
    </P>

    <P>Header messages are aligned on 8-byte boundaries.
    </P>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Object Headers
	</caption>

	<tr>
	  <th>byte</th>
	  <th>byte</th>
	  <th>byte</th>
	  <th>byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Reserved (zero)</td>
	  <td colspan=2>Number of Header Messages</td>
	</tr>

	<tr>
	  <td colspan=4>Object Reference Count</td>
	</tr>

	<tr>
	  <td colspan=4>Object Header Size</td>
	</tr>

	<tr>
	  <td colspan=2>Header Message Type #1</td>
	  <td colspan=2>Size of Header Message Data #1</td>
	</tr>

	<tr>
	  <td>Header Message #1 Flags</td>
	  <td colspan=3>Reserved (zero)</td>
	</tr>

	<tr>
	  <td colspan=4><br>Header Message Data #1<br><br></td>
	</tr>

	<tr>
	  <td colspan=4>.<br>.<br>.<br></td>
	</tr>

	<tr>
	  <td colspan=2>Header Message Type #n</td>
	  <td colspan=2>Size of Header Message Data #n</td>
	</tr>

	<tr>
	  <td>Header Message #n Flags</td>
	  <td colspan=3>Reserved (zero)</td>
	</tr>

	<tr>
	  <td colspan=4><br>Header Message Data #n<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>
            <P>This value is used to determine the format of the
	    information in the object header.  When the format of the
	    information in the object header is changed, the version number
	    is incremented and can be used to determine how the
	    information in the object header is formatted.  This
            document describes version one (1) (there was no version
            zero (0)).
            </P>
          </td>
	</tr>

	<tr>
	  <td>Number of Header Messages</td>
	  <td>
            <P>This value determines the number of messages listed in
	    object headers for this object.  This value includes the messages
            in continuation messages for this object.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Object Reference Count</td>
	  <td>
            <P>This value specifies the number of "hard links" to this object
            within the current file.  References to the object from external
            files, "soft links" in this file and object references in this
            file are not tracked.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Object Header Size</td>
	  <td>
            <P>This value specifies the number of bytes of header message data
            following this length field that contain object header messages
            for this object header.  This value does not include the size of
            object header continuation blocks for this object elsewhere in the
            file.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Header Message Type</td>
	  <td>
            <P>This value specifies the type of information included in the
            following header message data.  The header message types for the
            pre-defined header messages are included in sections below.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Size of Header Message Data</td>
	  <td>
            <P>This value specifies the number of bytes of header
	    message data following the header message type and length
	    information for the current message. The size includes
	    padding bytes to make the message a multiple of eight
	    bytes.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Header Message Flags</td>
	  <td>
            <P>This is a bit field with the following definition:
            <table class=list>
                <tr>
                  <th width="30%">Bit</th>
                  <th align=left>Description</th>
                </tr>

                <tr>
                  <td align=center><code>0</code></td>
                  <td>If set, the message data is constant.  This is used
                    for messages like the datatype message of a dataset.
                  </td>
                </tr>
                <tr>
                  <td align=center><code>1</code></td>
                  <td>If set, the message is stored in the global heap.
                    The Header Message Data field contains a Shared Object
                    message and the Size of Header Message Data field
                    contains the size of that Shared Object message.
                  </td>
                </tr>
                <tr>
                  <td align=center><code>2-7</code></td>
                  <td>Reserved</td>
                </tr>
	    </table>
            </P>
          </td>
        </tr>

	<tr>
	  <td>Header Message Data</td>
	  <td>
            <P>The format and length of this field is determined by the
	    header message type and size respectively.  Some header
	    message types do not require any data and this information
	    can be eliminated by setting the length of the message to
	    zero. The data is padded with enough zeros to make the
	    size a multiple of eight.
            </P>
          </td>
	</tr>
      </table>
    </div>

    <P>The header message types and the message data associated with
      them compose the critical "metadata" about each object.  Some
      header messages are required for each object while others are
      optional.  Some optional header messages may also be repeated
      several times in the header itself, the requirements and number
      of times allowed in the header will be noted in each header
      message description below.
    </P>

    <P>The following is a list of currently defined header messages:
    </P>

    <hr>
    <h4><a name="NILMessage">Name: NIL</a></h4>

    <P class=item><B>Header Message Type: </B>0x0000
    </P>
    <P class=item><B>Length:</B> varies
    </P>
    <P class=item><B>Status:</B> Optional, may be repeated.
    </P>
    <P class=item><B>Purpose and Description:</B> The NIL message is used to indicate a
        message which is to be ignored when reading the header messages for a
        data object.  [Possibly one which has been deleted for some reason.]
    </P>
    <P class=item><B>Format of Data:</B> Unspecified.
    </P>

    <hr>
    <h4><a name="SimpleDataSpace">Name: Simple Dataspace</a></h4>

    <P class=item><B>Header Message Type: </B>0x0001
    </P>
    <P class=item><B>Length:</B> Varies according to the number of dimensions,
      as described in the following table.
    </P>
    <P class=item><B>Status:</B> Required for dataset objects, may not be
        repeated.
    </P>
    <P class=item><B>Description:</B> The simple dataspace message describes the
        number of dimensions (i.e. "rank") and size of each dimension that the
        data object has.  This message is only used for datasets which have a
        simple, rectilinear grid layout; datasets requiring a more complex
        layout (irregularly structured or unstructured grids, etc.) must use
        the <em>Complex Dataspace</em> message for expressing the space the
        dataset inhabits.  <i>(Note: The <em>Complex Dataspace</em>
        functionality is not yet implemented and it is not described in this
        document.)</i>
    </P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Simple Dataspace Message
	</caption>

	<tr>
	  <th>byte</th>
	  <th>byte</th>
	  <th>byte</th>
	  <th>byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Dimensionality</td>
	  <td>Flags</td>
	  <td>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension #1 Size<sup>L</sup></td>
	<tr>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr>
	  <td colspan=4>Dimension #n Size<sup>L</sup></td>
	<tr>
	  <td colspan=4>Dimension #1 Maximum Size<sup>L</sup></td>
	<tr>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr>
	  <td colspan=4>Dimension #n Maximum Size<sup>L</sup></td>
	<tr>
	  <td colspan=4>Permutation Index #1<sup>L</sup></td>
	<tr>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr>
	  <td colspan=4>Permutation Index #n<sup>L</sup></td>
      </table>

      <table class=note>
        <tr><td>
            (Items marked with an 'L' the above table are
            <br>
            of the size specified in "Size of Lengths.")
        </td></tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>
            <P>This value is used to determine the format of the
	    Simple Dataspace Message.  When the format of the
	    information in the message is changed, the version number
	    is incremented and can be used to determine how the
	    information in the object header is formatted.  This
            document describes version one (1) (there was no version
            zero (0)).
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td>
            <P>This value is the number of dimensions that the data
	    object has.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Flags</td>
	  <td>
            <P>This field is used to store flags to indicate the
	    presence of parts of this message.  Bit 0 (the least
	    significant bit) is used to indicate that maximum
	    dimensions are present.  Bit 1 is used to indicate that
	    permutation indices are present.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimension #n Size</td>
	  <td>
            <P>This value is the current size of the dimension of the
	    data as stored in the file.  The first dimension stored in
	    the list of dimensions is the slowest changing dimension
	    and the last dimension stored is the fastest changing
	    dimension.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimension #n Maximum Size</td>
	  <td>
            <P>This value is the maximum size of the dimension of the
	    data as stored in the file.  This value may be the special
            "<A href="#UnlimitedDim">unlimited</A>" size which indicates
	    that the data may expand along this dimension indefinitely.
            If these values are not stored, the maximum size of each
            dimension is assumed to be the dimension's current size.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Permutation Index #n</td>
	  <td>
            <P>This value is the index permutation used to map
	    each dimension from the canonical representation to an
	    alternate axis for each dimension.  If these values are
	    not stored, the first dimension stored in the list of
	    dimensions is the slowest changing dimension and the last
	    dimension stored is the fastest changing dimension.
            </P>
          </td>
	</tr>
      </table>
    </div>

    </P>

<!--
    <hr>
    <h4><a name="DataSpaceMessage">Name: Complex Dataspace (Fiber Bundle?)</a></h4>
    <b>Header Message Type: </b>0x0002<br>
    <b>Length:</b> varies<br>

    <b>Status:</b> One of the <em>Simple Dataspace</em> or
    <em>Complex Dataspace</em> messages is required (but not both) and may
    not be repeated.<br> <b>Purpose and Description:</b> The
    <em>Dataspace</em> message describes space that the dataset is
    mapped onto in a more comprehensive way than the <em>Simple
    Dimensionality</em> message is capable of handling.  The
    dataspace of a dataset encompasses the type of coordinate system
    used to locate the dataset's elements as well as the structure and
    regularity of the coordinate system.  The dataspace also
    describes the number of dimensions which the dataset inhabits as
    well as a possible higher dimensional space in which the dataset
    is located within.

    <br>
    <b>Format of Data:</b>

    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Dataspace Message Layout</B>
	</caption>

	<tr align=center>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>

	<tr align=center>
	  <td colspan=4>Mesh Type</td>
	<tr align=center>
	  <td colspan=4>Logical Dimensionality</td>
      </table>
    </center>

    <p>
    <dl>
      <dt>The elements of the dimensionality message are described below:
      <dd>
	<dl>
	  <dt>Mesh Type: (unsigned 32-bit integer)
	  <dd>This value indicates whether the grid is
	    polar/spherical/cartesion,
	    structured/unstructured and regular/irregular. <br>
	    The mesh type value is broken up as follows: <br>

	    <P>
	    <center>
	      <table border cellpadding=4 width="80%">
		<caption align=bottom>
		  <B>HDF5 Mesh-type Layout</B>
		</caption>

		<tr align=center>
		  <th width="25%">byte</th>
		  <th width="25%">byte</th>
		  <th width="25%">byte</th>
		  <th width="25%">byte</th>

		<tr align=center>
		  <td colspan=1>Mesh Embedding</td>
		  <td colspan=1>Coordinate System</td>
		  <td colspan=1>Structure</td>
		  <td colspan=1>Regularity</td>
	      </table>
	    </center>
	    The following are the definitions of mesh-type bytes:
	    <dl>
	      <dt>Mesh Embedding
	      <dd>This value indicates whether the dataset dataspace
		is located within
		another dataspace or not:
		<dl> <dl>
		    <dt>&lt;STANDALONE&gt;
		    <dd>The dataset mesh is self-contained and is not
		      embedded in another mesh.
		    <dt>&lt;EMBEDDED&gt;
		    <dd>The dataset's dataspace is located within
		      another dataspace, as
		      described in information below.
		  </dl> </dl>
	      <dt>Coordinate System
	      <dd>This value defines the type of coordinate system
		used for the mesh:
		<dl> <dl>
		    <dt>&lt;POLAR&gt;
		    <dd>The last two dimensions are in polar
		      coordinates, higher dimensions are
		      cartesian.
		    <dt>&lt;SPHERICAL&gt;
		    <dd>The last three dimensions are in spherical
		      coordinates, higher dimensions
		      are cartesian.
		    <dt>&lt;CARTESIAN&gt;
		    <dd>All dimensions are in cartesian coordinates.
		  </dl> </dl>
	      <dt>Structure
	      <dd>This value defines the locations of the grid-points
		on the axes:
		<dl> <dl>
		    <dt>&lt;STRUCTURED&gt;
		    <dd>All grid-points are on integral, sequential
		      locations, starting from 0.
		    <dt>&lt;UNSTRUCTURED&gt;
		    <dd>Grid-points locations in each dimension are
		      explicitly defined and
		      may be of any numeric datatype.
		  </dl> </dl>
	      <dt>Regularity
	      <dd>This value defines the locations of the dataset
		points on the grid:
		<dl> <dl>
		    <dt>&lt;REGULAR&gt;
		    <dd>All dataset elements are located at the
		      grid-points defined.
		    <dt>&lt;IRREGULAR&gt;
		    <dd>Each dataset element has a particular
		      grid-location defined.
		  </dl> </dl>
	    </dl>
	    <p>The following grid combinations are currently allowed:
	    <dl> <dl>
		<dt>&lt;POLAR-STRUCTURED-REGULAR&gt;
		<dt>&lt;SPHERICAL-STRUCTURED-REGULAR&gt;
		<dt>&lt;CARTESIAN-STRUCTURED-REGULAR&gt;
		<dt>&lt;POLAR-UNSTRUCTURED-REGULAR&gt;
		<dt>&lt;SPHERICAL-UNSTRUCTURED-REGULAR&gt;
		<dt>&lt;CARTESIAN-UNSTRUCTURED-REGULAR&gt;
		<dt>&lt;CARTESIAN-UNSTRUCTURED-IRREGULAR&gt;
	      </dl> </dl>
	    All of the above grid types can be embedded within another
	    dataspace.
	    <br> <br>
	  <dt>Logical Dimensionality: (unsigned 32-bit integer)
	  <dd>This value is the number of dimensions that the dataset occupies.

	    <P>
	    <center>
	      <table border cellpadding=4 width="80%">
		<caption align=bottom>
		  <B>HDF5 Dataspace Embedded Dimensionality Information</B>
		</caption>

		<tr align=center>
		  <th width="25%">byte</th>
		  <th width="25%">byte</th>
		  <th width="25%">byte</th>
		  <th width="25%">byte</th>

		<tr align=center>
		  <td colspan=4>Embedded Dimensionality</td>
		<tr align=center>
		  <td colspan=4>Embedded Dimension Size #1</td>
		<tr align=center>
		  <td colspan=4>.<br>.<br>.<br></td>
		<tr align=center>
		  <td colspan=4>Embedded Dimension Size #n</td>
		<tr align=center>
		  <td colspan=4>Embedded Origin Location #1</td>
		<tr align=center>
		  <td colspan=4>.<br>.<br>.<br></td>
		<tr align=center>
		  <td colspan=4>Embedded Origin Location #n</td>
	      </table>
	    </center>

	  <dt>Embedded Dimensionality: (unsigned 32-bit integer)
	  <dd>This value is the number of dimensions of the space the
	    dataset is located
	    within.  i.e. a planar dataset located within a 3-D space,
	    or a 3-D dataset
	    which is a subset of another 3-D space, etc.
	  <dt>Embedded Dimension Size: (unsigned 32-bit integer)
	  <dd>These values are the sizes of the dimensions of the
	    embedded dataspace
	    that the dataset is located within.
	  <dt>Embedded Origin Location: (unsigned 32-bit integer)
	  <dd>These values comprise the location of the dataset's
	    origin within the embedded dataspace.
	</dl>
    </dl>
    [Comment: need some way to handle different orientations of the
    dataset dataspace
    within the embedded dataspace]<br>

    <P>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Dataspace Structured/Regular Grid Information</B>
	</caption>

	<tr align=center>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>

	<tr align=center>
	  <td colspan=4>Logical Dimension Size #1</td>
	<tr align=center>
	  <td colspan=4>Logical Dimension Maximum #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Logical Dimension Size #n</td>
	<tr align=center>
	  <td colspan=4>Logical Dimension Maximum #n</td>
      </table>
    </center>

    <p>
    <dl>
      <dt>The elements of the dimensionality message are described below:
      <dd>
	<dl>
	  <dt>Logical Dimension Size #n: (unsigned 32-bit integer)
	  <dd>This value is the current size of the dimension of the
	    data as stored in
	    the file.  The first dimension stored in the list of
	    dimensions is the slowest
	    changing dimension and the last dimension stored is the
	    fastest changing
	    dimension.
	  <dt>Logical Dimension Maximum #n: (unsigned 32-bit integer)
	  <dd>This value is the maximum size of the dimension of the
	    data as stored in
	    the file.  This value may be the special value
	    &lt;UNLIMITED&gt; which
	    indicates that the data may expand along this dimension
	    indefinitely.
	</dl>
    </dl>
    <P>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Dataspace Structured/Irregular Grid Information</B>
	</caption>

	<tr align=center>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>

	<tr align=center>
	  <td colspan=4># of Grid Points in Dimension #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4># of Grid Points in Dimension #n</td>
	<tr align=center>
	  <td colspan=4>Datatype of Grid Point Locations</td>
	<tr align=center>
	  <td colspan=4>Location of Grid Points in Dimension #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Location of Grid Points in Dimension #n</td>
      </table>
    </center>

    <P>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Dataspace Unstructured Grid Information</B>
	</caption>

	<tr align=center>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>

	<tr align=center>
	  <td colspan=4># of Grid Points</td>
	<tr align=center>
	  <td colspan=4>Datatype of Grid Point Locations</td>
	<tr align=center>
	  <td colspan=4>Grid Point Locations<br>.<br>.<br></td>
      </table>
    </center>
-->

    <hr>
    <h4><a name="ReservedMessage_0002">Name: Reserved - Not Assigned Yet</a></h4>
    <b>Header Message Type:</b> 0x0002<BR>
    <b>Length:</b> N/A<BR>
    <b>Status:</b> N/A<BR>
    <b>Format of Data:</b> N/A<BR>

    <p><b>Purpose and Description:</b> This message type was skipped during
      the initial specification of the file format and may be used in a
      future expansion to the format.


    <hr>
    <h4><a name="DataTypeMessage">Name: Datatype</a></h4>

    <P class=item><B>Header Message Type:</B> 0x0003
    </P>
    <P class=item><B>Length:</B> variable
    </P>
    <P class=item><B>Status:</B> Required for dataset or named datatype objects,
        may not be repeated.
    </P>

    <P class=item><B>Description:</B> The datatype message defines the datatype
        for each element of a dataset.  A datatype can describe an atomic type
        like a fixed- or floating-point type or a compound type like a C
        struct.
        Datatypes messages are stored
        as a list of datatype classes and
        their associated properties.
    </P>

    <P class=item2>Datatype messages that are part of a dataset object,
        do not describe how elements are related to one another, the dataspace
        message is used for that purpose.  Datatype messages that are part of
        a named datatype message describe an "abstract" datatype that can be
        used by other objects in the file.
    </P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Datatype Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Class and Version</td>
	  <td>Class Bit Field, Bits 0-7</td>
	  <td>Class Bit Field, Bits 8-15</td>
	  <td>Class Bit Field, Bits 16-23</td>
	</tr>

	<tr>
	  <td colspan=4>Size</td>
	</tr>

	<tr>
	  <td colspan=4><br><br>Properties<br><br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Class and Version</td>
	  <td>
            <P>The version of the datatype message and the datatype's class
                information are packed together in this field.  The version
                number is packed in the top 4 bits of the field and the class
                is contained in the bottom 4 bits.
            </P>
            <P>The version number information is used for changes in the
                format of the datatype message and is described here:
                <table class=list>
                    <tr>
                      <th width="30%">Version</th>
                      <th align=left>Description</th>
                    </tr>

                    <tr>
                      <td align=center><code>0</code></td>
                      <td>Never used
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>1</code></td>
                      <td>Used by early versions of the library to encode
                            compound datatypes with explicit array fields.
                            See the compound datatype description below for
                            further details.
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>2</code></td>
                      <td>The current version used by the library.
                      </td>
                    </tr>
                </table>
            </P>
            <P>The class of the datatype determines the format for the class
                bit field and properties portion of the datatype message, which
                are described below.  The
                following classes are currently defined:
                <table width=100% class=list>
                    <tr>
                      <th width="30%">Value</th>
                      <th align=left>Description</th>
                    </tr>

                    <tr>
                      <td align=center><code>0</code></td>
                      <td>Fixed-Point</td>
                    </tr>

                    <tr>
                      <td align=center><code>1</code></td>
                      <td>Floating-Point</td>
                    </tr>

                    <tr>
                      <td align=center><code>2</code></td>
                      <td>Time</td>
                    </tr>

                    <tr>
                      <td align=center><code>3</code></td>
                      <td>String</td>
                    </tr>

                    <tr>
                      <td align=center><code>4</code></td>
                      <td>Bitfield</td>
                    </tr>

                    <tr>
                      <td align=center><code>5</code></td>
                      <td>Opaque</td>
                    </tr>

                    <tr>
                      <td align=center><code>6</code></td>
                      <td>Compound</td>
                    </tr>

                    <tr>
                      <td align=center><code>7</code></td>
                      <td>Reference</td>
                    </tr>

                    <tr>
                      <td align=center><code>8</code></td>
                      <td>Enumerated</td>
                    </tr>

                    <tr>
                      <td align=center><code>9</code></td>
                      <td>Variable-Length</td>
                    </tr>

                    <tr>
                      <td align=center><code>10</code></td>
                      <td>Array</td>
                    </tr>
                </table>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Class Bit Fields</td>
	  <td>
            <P>The information in these bit fields is specific to each datatype
                class and is described below.  All bits not defined for a
                datatype class are set to zero.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Size</td>
	  <td>
            <P>The size of the datatype in bytes.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Properties</td>
	  <td>
            <P>This variable-sized field encodes information specific to each
                datatype class and is described below.  If there is no
                property information specified for a datatype class, the size
                of this field is zero.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>

    <P>Class specific information for Fixed-Point Numbers (Class 0):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr>
	  <td>1, 2</td>
	  <td><b>Padding type.</b>  Bit 1 is the lo_pad type and bit 2
	    is the hi_pad type.  If a datum has unused bits at either
	    end, then the lo_pad or hi_pad bit is copied to those
	    locations.</td>
	</tr>

	<tr>
	  <td>3</td>
	  <td><b>Signed.</b> If this bit is set then the fixed-point
	    number is in 2's complement form.</td>
	</tr>

	<tr>
	  <td>4-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Descriptions
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=2>Bit Offset</td>
	  <td colspan=2>Bit Precision</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Bit Offset</td>
	  <td>
            <P>The bit offset of the first significant bit of the fixed-point
                value within the datatype.  The bit offset specifies the number
                of bits "to the right of" the value.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Bit Precision</td>
	  <td>
            <P>The number of bits of precision of the fixed-point value
                within the datatype.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>

    <P>Class specific information for Floating-Point Numbers (Class 1):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr>
	  <td>1, 2, 3</td>
	  <td><b>Padding type.</b>  Bit 1 is the low bits pad type, bit 2
	    is the high bits pad type, and bit 3 is the internal bits
	    pad type.  If a datum has unused bits at either end or between
	    the sign bit, exponent, or mantissa, then the value of bit
	    1, 2, or 3 is copied to those locations.</td>
	</tr>

	<tr>
	  <td>4-5</td>
	  <td><b>Normalization.</b> The value can be 0 if there is no
	    normalization, 1 if the most significant bit of the
	    mantissa is always set (except for 0.0), and 2 if the most
	    significant bit of the mantissa is not stored but is
	    implied to be set. The value 3 is reserved and will not
	    appear in this field.</td>
	</tr>

	<tr>
	  <td>6-7</td>
	  <td>Reserved (zero).</td>
	</tr>

	<tr>
	  <td>8-15</td>
	  <td><b>Sign Location.</b> This is the bit position of the sign
	    bit.  Bits are numbered with the least significant bit zero.</td>
	</tr>

	<tr>
	  <td>16-23</td>
	  <td>Reserved (zero).</td>
	</tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Descriptions
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=2>Bit Offset</td>
	  <td colspan=2>Bit Precision</td>
	</tr>

	<tr>
	  <td>Exponent Location</td>
	  <td>Exponent Size</td>
	  <td>Mantissa Location</td>
	  <td>Mantissa Size</td>
	</tr>

	<tr>
	  <td colspan=4>Exponent Bias</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Bit Offset</td>
	  <td>
            <P>The bit offset of the first significant bit of the floating-point
                value within the datatype.  The bit offset specifies the number
                of bits "to the right of" the value.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Bit Precision</td>
	  <td>
            <P>The number of bits of precision of the floating-point value
                within the datatype.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Exponent Location</td>
	  <td>
            <P>The bit position of the exponent field.  Bits are numbered with
                the least significant bit number zero.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Exponent Size</td>
	  <td>
            <P>The size of the exponent field in bits.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Mantissa Location</td>
	  <td>
            <P>The bit position of the mantissa field.  Bits are numbered with
                the least significant bit number zero.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Mantissa Size</td>
	  <td>
            <P>The size of the mantissa field in bits.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Exponent Bias</td>
	  <td>
            <P>The bias of the exponent field.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>

    <P>Class specific information for Time (Class 2):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr>
	  <td>1-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Descriptions
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=2>Bit Precision</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Bit Precision</td>
	  <td>
            <P>The number of bits of precision of the time value.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>

    <P>Class specific information for Strings (Class 3):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0-3</td>
	  <td><b>Padding type.</b>  This four-bit value determines the
	    type of padding to use for the string.  The values are:

            <table width=100% class=list>
                <tr>
                  <th width="30%">Value</th>
                  <th align=left>Description</th>
                </tr>

                <tr>
                  <td align=center><code>0</code></td>
                  <td>Null Terminate: A zero byte marks the end of the
                    string and is guaranteed to be present after
                    converting a long string to a short string.  When
                    converting a short string to a long string the value is
                    padded with additional null characters as necessary.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>1</code></td>
                  <td>Null Pad: Null characters are added to the end of
                    the value during conversions from short values to long
                    values but conversion in the opposite direction simply
                    truncates the value.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>2</code></td>
                  <td>Space Pad: Space characters are added to the end of
                    the value during conversions from short values to long
                    values but conversion in the opposite direction simply
                    truncates the value.  This is the Fortran
                    representation of the string.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>3-15</code></td>
                  <td>Reserved
                  </td>
                </tr>
            </table>
	</tr>

	<tr>
	  <td>4-7</td>
	  <td><b>Character Set.</b>  The character set to use for
	    encoding the string.  The only character set supported is
	    the 8-bit ASCII (zero) so no translations have been defined
	    yet.</td>
	</tr>

	<tr>
	  <td>8-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <P>There are no properties defined for the string class.
    </P>
    </P>

    <P>Class specific information for Bitfields (Class 4):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr>
	  <td>1, 2</td>
	  <td><b>Padding type.</b>  Bit 1 is the lo_pad type and bit 2
	    is the hi_pad type.  If a datum has unused bits at either
	    end, then the lo_pad or hi_pad bit is copied to those
	    locations.</td>
	</tr>

	<tr>
	  <td>3-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Description
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=2>Bit Offset</td>
	  <td colspan=2>Bit Precision</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Bit Offset</td>
	  <td>
            <P>The bit offset of the first significant bit of the bitfield
                within the datatype.  The bit offset specifies the number
                of bits "to the right of" the value.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Bit Precision</td>
	  <td>
            <P>The number of bits of precision of the bitfield
                within the datatype.
            </P>
          </td>
	</tr>
      </table>
    </div>
    </P>

    <P>Class specific information for Opaque (Class 5):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0-7</td>
	  <td>Length of ASCII tag in bytes.</td>
	</tr>

	<tr>
	  <td>8-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Description
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>ASCII Tag<br>
	    <br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>ASCII Tag</td>
	  <td>
            <P>This NUL-terminated string provides a description for the
                opaque type.  It is NUL-padded to a multiple of 8 bytes.
            </P>
          </td>
	</tr>
      </table>
    </div>
    </P>

    <P>Class specific information for Compound (Class 6):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0-15</td>
	  <td><b>Number of Members.</b> This field contains the number
	    of members defined for the compound datatype.  The member
	    definitions are listed in the Properties field of the data
	    type message.
	</tr>

	<tr>
	  <td>15-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>
    </P>

    <P>The Properties field of a compound datatype is a list of the
      member definitions of the compound datatype.  The member
      definitions appear one after another with no intervening bytes.
      The member types are described with a recursive datatype
      message.

    <P>Note that the property descriptions are different for different
      versions of the datatype version.  Additionally note that the version
      0 properties are deprecated and have been replaced with the version
      1 properties in versions of the HDF5 library from the 1.4 release
      onward.

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Properties Description for Datatype Version 1
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr>
	  <td colspan=4>Byte Offset of Member</td>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td colspan=3>Reserved (zero)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension Permutation</td>
	</tr>

	<tr>
	  <td colspan=4>Reserved (zero)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension #1 Size (required)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension #2 Size (required)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension #3 Size (required)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension #4 Size (required)</td>
	</tr>

	<tr>
	  <td colspan=4><br>Member Type Message<br><br></td>
	</tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Name</td>
	  <td>
            <P>This NUL-terminated string provides a description for the
                opaque type.  It is NUL-padded to a multiple of 8 bytes.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Byte Offset of Member</td>
	  <td>
            <P>This is the byte offset of the member within the datatype.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td>
            <P>If set to zero, this field indicates a scalar member.  If set
                to a value greater than zero, this field indicates that the
                member is an array of values.  For array members, the size of
                the array is indicated by the 'Size of Dimension n' field in
                this message.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimension Permutation</td>
	  <td>
            <P>This field was intended to allow an array field to have
                it's dimensions permuted, but this was never implemented.
                This field should always be set to zero.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimension #n Size</td>
	  <td>
            <P>This field is the size of a dimension of the array field as
            stored in the file.  The first dimension stored in the list of
            dimensions is the slowest changing dimension and the last
            dimension stored is the fastest changing dimension.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Member Type Message</td>
	  <td>
            <P>This field is a datatype message describing the datatype of
            the member.
            </P>
          </td>
	</tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Properties Description for Datatype Version 2
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr>
	  <td colspan=4>Byte Offset of Member</td>
	</tr>

	<tr>
	  <td colspan=4><br>Member Type Message<br><br></td>
	</tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Name</td>
	  <td>
            <P>This NUL-terminated string provides a description for the
                opaque type.  It is NUL-padded to a multiple of 8 bytes.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Byte Offset of Member</td>
	  <td>
            <P>This is the byte offset of the member within the datatype.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Member Type Message</td>
	  <td>
            <P>This field is a datatype message describing the datatype of
            the member.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>

    <P>Class specific information for Reference (Class 7):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0-3</td>
	  <td><b>Type.</b> This four-bit value contains the type of reference
            described.  The values defined are:

            <table width=100% class=list>
                <tr>
                  <th width="30%">Value</th>
                  <th align=left>Description</th>
                </tr>

                <tr>
                  <td align=center><code>0</code></td>
                  <td>Object Reference: A reference to another object in this
                    HDF5 file.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>1</code></td>
                  <td>Dataset Region Reference: A reference to a region within
                    a dataset in this HDF5 file.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>2</code></td>
                  <td>Internal Reference: A reference to a region within the
                    current dataset.  (Not currently implemented)
                  </td>
                </tr>

                <tr>
                  <td align=center><code>3-15</code></td>
                  <td>Reserved
                  </td>
                </tr>
            </table>

            </td>
	</tr>

	<tr>
	  <td>15-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <P>There are no properties defined for the reference class.
    </P>
    </P>

    <P>Class specific information for Enumeration (Class 8):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0-15</td>
	  <td><b>Number of Members.</b> The number of name/value
	    pairs defined for the enumeration type.</td>
	</tr>

	<tr>
	  <td>16-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Description
	</caption>

	<tr>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	  <th width="25%">Byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>Base Type<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Names<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Values<br><br></td>
        </tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Base Type</td>
	  <td>
            <P>Each enumeration type is based on some parent type, usually an
                integer. The information for that parent type is described
                recursively by this field.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Names</td>
	  <td>
            <P>The name for each name/value pair. Each name is stored as a null
                terminated ASCII string in a multiple of eight bytes. The names
                are in no particular order.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Values</td>
	  <td>
            <P>The list of values in the same order as the names.  The values
                are packed (no inter-value padding) and the size of each value
                is determined by the parent type.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>


    <P>Class specific information for Variable-Length (Class 9):

    <br>
    <div align=center>
      <table class=desc>
	<caption>
	  Bit Field Description
	</caption>

	<tr>
	  <th width="10%">Bits</th>
	  <th>Meaning</th>
	</tr>

	<tr>
	  <td>0-3</td>
	  <td><b>Type.</b> This four-bit value contains the type of
            variable-length datatype described.  The values defined are:

            <table width=100% class=list>
                <tr>
                  <th width="30%">Value</th>
                  <th align=left>Description</th>
                </tr>

                <tr>
                  <td align=center><code>0</code></td>
                  <td>Sequence: A variable-length sequence of any sequence of
                    data.  Variable-length sequences do not have padding or
                    character set information.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>1</code></td>
                  <td>String: A variable-length sequence of characters.
                    Variable-length strings have padding and character set
                    information.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>2-15</code></td>
                  <td>Reserved
                  </td>
                </tr>
            </table>

            </td>
	</tr>

	<tr>
	  <td>4-7</td>
	  <td><b>Padding type.</b> (variable-length string only)
                  This four-bit value determines the type of padding
                  used for variable-length strings.  The values are the same
                  as for the string padding type, as follows:
            <table width=100% class=list>
                <tr>
                  <th width="30%">Value</th>
                  <th align=left>Description</th>
                </tr>

                <tr>
                  <td align=center><code>0</code></td>
                  <td>Null terminate: A zero byte marks the end of a string
                    and is guaranteed to be present after converting a long
                    string to a short string.  When converting a short string
                    to a long string, the value is padded with additional null
                    characters as necessary.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>1</code></td>
                  <td>Null pad: Null characters are added to the end of the
                    value during conversion from a short string to a longer
                    string.  Conversion from a long string to a shorter string
                    simply truncates the value.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>2</code></td>
                  <td>Space pad: Space characters are added to the end of the
                    value during conversion from a short string to a longer
                    string.  Conversion from a long string to a shorter string
                    simply truncates the value.  This is the Fortran
                    representation of the string.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>3-15</code></td>
                  <td>Reserved
                  </td>
                </tr>
            </table>

            This value is set to zero for variable-length sequences.

          </td>
	</tr>

	<tr>
	  <td>8-11</td>
	  <td><b>Character Set.</b> (variable-length string only)
                  This four-bit value specifies the character set
                  to be used for encoding the string:
            <table width=100% class=list>
                <tr>
                  <th width="30%">Value</th>
                  <th align=left>Description</th>
                </tr>

                <tr>
                  <td align=center><code>0</code></td>
                  <td>ASCII: As of this writing (July 2003, Release 1.6.0),
                    8-bit ASCII is the only character set supported.  Therefore,
                    no translations have been defined.
                  </td>
                </tr>

                <tr>
                  <td align=center><code>1-15</code></td>
                  <td>Reserved
                  </td>
                </tr>
            </table>

            This value is set to zero for variable-length sequences.

          </td>
	</tr>

	<tr>
	  <td>12-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Property Description
        </caption>

        <tr>
          <th width="25%">Byte</th>
          <th width="25%">Byte</th>
          <th width="25%">Byte</th>
          <th width="25%">Byte</th>
        </tr>

        <tr>
          <td colspan=4><br>Base Type<br><br></td>
        </tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Base Type</td>
	  <td>
            <P>Each variable-length type is based on some parent type.  The
                information for that parent type is described recursively by
                this field.
            </P>
          </td>
	</tr>

      </table>
    </div>
    </P>

    <P>Class specific information for Array (Class 10):

    <P>There are no bit fields defined for the array class.
    </P>

    <P>Note that the dimension information defined in the property for this
      datatype class is independent of dataspace information for a dataset.
      The dimension information here describes the dimensionality of the
      information within a data element (or a component of an element, if the
      array datatype is nested within another datatype) and the dataspace for a
      dataset describes the location of the elements in a dataset.
    </P>

    <br>
    <div align=center>
      <table class=format>
        <caption>
          Property Description
        </caption>

        <tr>
          <th width="25%">Byte</th>
          <th width="25%">Byte</th>
          <th width="25%">Byte</th>
          <th width="25%">Byte</th>
        </tr>

	<tr>
	  <td>Dimensionality</td>
	  <td colspan=3>Reserved (zero)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension #1 Size</td>
	</tr>
	<tr>
	  <td colspan=4>.<br>.<br>.<br></td>
	</tr>
	<tr>
	  <td colspan=4>Dimension #n Size</td>
	</tr>

	<tr>
	  <td colspan=4>Permutation Index #1</td>
	</tr>
	<tr>
	  <td colspan=4>.<br>.<br>.<br></td>
	</tr>
	<tr>
	  <td colspan=4>Permutation Index #n</td>
	</tr>

        <tr>
          <td colspan=4><br>Base Type<br><br></td>
        </tr>

      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td>
            <P>This value is the number of dimensions that the array has.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dimension #n Size</td>
	  <td>
            <P>This value is the size of the dimension of the array
	    as stored in the file.  The first dimension stored in
	    the list of dimensions is the slowest changing dimension
	    and the last dimension stored is the fastest changing
	    dimension.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Permutation Index #n</td>
	  <td>
            <P>This value is the index permutation used to map
	    each dimension from the canonical representation to an
	    alternate axis for each dimension.  Currently, dimension
            permutations are not supported and these indices should be set
            to the index position minus one (i.e. the first dimension should
            be set to 0, the second dimension should be set to 1, etc.)
            </P>
          </td>
	</tr>

	<tr>
	  <td>Base Type</td>
	  <td>
            <P>Each array type is based on some parent type.  The
                information for that parent type is described recursively by
                this field.
            </P>
          </td>
	</tr>

      </table>
    </div>

    </P>

    <hr>
    <h4><a name="OldFillValueMessage">Name: Data Storage - Fill Value (Old)</a></h4>

    <P class=item><B>Header Message Type:</B> 0x0004
    </P>
    <P class=item><B>Length:</B> varies
    </P>
    <P class=item><B>Status:</B> Optional, may not be repeated.
    </P>

    <P class=item><B>Description:</B> The fill value message stores a single
        data value which is returned to the application when an uninitialized
        data element is read from a dataset.  The fill value is interpreted
        with the same datatype as the dataset.  If no fill value message is
        present then a fill value of all zero bytes is assumed.
    </P>

    <P class=item2>This fill value message is deprecated in favor of the "new"
        fill value message (Message Type 0x0005) and is only written to the
        file for forward compatibility with versions of the HDF5 library before
        the 1.6.0 version.  Additionally, it only appears for datasets with a
        user defined fill value (as opposed to the library default fill value
        or an explicitly set "undefined" fill value).
    </P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Fill Value Message (Old)
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=4>Size</td>
	</tr>

	<tr>
	  <td colspan=4><br>Fill Value<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Size</td>
	  <td>
            <P>This is the size of the Fill Value field in bytes.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Fill Value</td>
	  <td>
            <P>The fill value.  The bytes of the fill value are interpreted
                using the same datatype as for the dataset.
            </P>
          </td>
	</tr>
      </table>
    </div>
    </P>

    <hr>
    <h4><a name="FillValueMessage">Name: Data Storage - Fill Value </a></h4>

    <P class=item><B>Header Message Type:</B> 0x0005
    </P>
    <P class=item><B>Length:</B> varies
    </P>
    <P class=item><B>Status:</B> Required for dataset objects, may not be repeated.
    </P>

    <P class=item><B>Description:</B> The fill value message stores a single
        data value which is returned to the application when an uninitialized
        data element is read from a dataset.  The fill value is interpreted
        with the same datatype as the dataset.
    </P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Fill Value Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Space Allocation Time</td>
	  <td>Fill Value Write Time</td>
	  <td>Fill Value Defined</td>
	</tr>

	<tr>
	  <td colspan=4>Size</td>
	</tr>

	<tr>
	  <td colspan=4><br>Fill Value<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>
            <P>The version number information is used for changes in the
                format of the fill value message and is described here:
                <table class=list>
                    <tr>
                      <th width="30%">Version</th>
                      <th align=left>Description</th>
                    </tr>

                    <tr>
                      <td align=center><code>0</code></td>
                      <td>Never used
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>1</code></td>
                      <td>Used by version 1.6.x of the library to encode
                            fill values.  In this version, the Size field is
                            always present.
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>2</code></td>
                      <td>The current version used by the library (version
                        1.7.3 or later).  In this version, the Size and
                        Fill Value fields are
                        only present if the Fill Value Defined field is set
                        to 1.
                      </td>
                    </tr>
                </table>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Space Allocation Time</td>
	  <td>
            <P>When the storage space for the dataset's raw data will be
                allocated.  The allowed values are:
                <table class=list>
                    <tr>
                      <th width="30%">Value</th>
                      <th align=left>Description</th>
                    </tr>

                    <tr>
                      <td align=center><code>1</code></td>
                      <td>Early allocation.  Storage space for the entire dataset
                        should be allocated in the file when the dataset is
                        created.
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>2</code></td>
                      <td>Late allocation.  Storage space for the entire dataset
                        should not be allocated until the dataset is written
                        to.
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>3</code></td>
                      <td>Incremental allocation.  Storage space for the
                        dataset should not be allocated until the portion
                        of the dataset is written to.  This is currently
                        used in conjunction with chunked data storage for
                        datasets.
                      </td>
                    </tr>
                </table>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Fill Value Write Time</td>
	  <td>
            <P>At the time that storage space for the dataset's raw data is
                allocated, this value indicates whether the fill value should
                be written to the raw data storage elements.  The allowed values
                are:
                <table class=list>
                    <tr>
                      <th width="30%">Value</th>
                      <th align=left>Description</th>
                    </tr>

                    <tr>
                      <td align=center><code>0</code></td>
                      <td>On allocation.  The fill value is always written to
                        the raw data storage when the storage space is allocated.
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>1</code></td>
                      <td>Never.  The fill value should never be written to
                        the raw data storage.
                      </td>
                    </tr>
                    <tr>
                      <td align=center><code>2</code></td>
                      <td>Fill value written if set by user.  The fill value
                        will be written to the raw data storage when the storage
                        space is allocated only if the user explicitly set
                        the fill value.  If the fill value is the library
                        default or is undefined, it will not be written to
                        the raw data storage.
                      </td>
                    </tr>
                </table>
            </P>
          </td>
	</tr>

	<tr>
	  <td>Fill Value Defined</td>
	  <td>
            <P>This value indicates if a fill value is defined for this
                dataset.  If this value is 0, the fill value is undefined.
                If this value is 1, a fill value is defined for this dataset.
                For version 2 or later of the fill value message, this value
                controls the presence of the Size field.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Size</td>
	  <td>
            <P>This is the size of the Fill Value field in bytes.  This field
                is not present if the Version field is >1 and the Fill Value
                Defined field is set to 0.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Fill Value</td>
	  <td>
            <P>The fill value.  The bytes of the fill value are interpreted
                using the same datatype as for the dataset.  This field is
                not present if the Version field is >1 and the Fill Value
                Defined field is set to 0.
            </P>
          </td>
	</tr>
      </table>
    </div>
    </P>

<!--
    <hr>
    <h4><a name="CompactDataStorageMessage">Name: Data Storage - Compact</a></h4>

    <b>Header Message Type:</b> 0x0006<br>
    <b>Length:</b> varies<br>
    <b>Status:</b> Optional, may not be repeated.<br>

    <p>This message indicates that the data for the data object is
      stored within the current HDF file by including the actual
      data as the header data for this message.  The data is
      stored internally in
      the <em>normal format</em>, i.e. in one chunk, uncompressed, etc.

    <P>Note that one and only one of the <em>Data Storage</em> headers can be
      stored for each data object.

    <P><b>Format of Data:</b>  The message data is actually composed
      of dataset data, so the format will be determined by the dataset
      format.
-->

    <hr>
    <h4><a name="ReservedMessage_0006">Name: Reserved - Not Assigned Yet</a></h4>
    <P class=item><B>Header Message Type:</B> 0x0006</P>
    <P class=item><B>Length:</B> N/A</P>
    <P class=item><B>Status:</B> N/A</P>
    <P class=item><B>Format of Data:</B> N/A</P>

    <P class=item><B>Purpose and Description:</B> This message type was skipped during
      the initial specification of the file format and may be used in a
      future expansion to the format.</P>

    <hr>
    <h4><a name="ExternalFileListMessage">Name: Data Storage -
	External Data Files</a></h4>
    <P class=item><B>Header Message Type:</B> 0x0007 </P>
    <P class=item><B>Length:</B> varies</P>
    <P class=item><B>Status:</B> Optional, may not be repeated.</P>

    <P class=item><B>Purpose and Description:</B> The external object message
      indicates that the data for an object is stored outside the HDF5
      file.  The filename of the object is stored as a Universal
      Resource Location (URL) of the actual filename containing the
      data. An external file list record also contains the byte offset
      of the start of the data within the file and the amount of space
      reserved in the file for that data.</P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  External File List Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td colspan=3>Reserved</td>
	</tr>

	<tr>
	  <td colspan=2>Allocated Slots</td>
	  <td colspan=2>Used Slots</td>
	</tr>

	<tr>
	  <td colspan=4><br>Heap Address<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Slot Definitions...<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
	<table class=desc>
	  <tr>
	    <th width="30%">Field Name</th>
	    <th>Description</th>
	  </tr>

	  <tr>
	    <td>Version</td>
	    <td>
	      <P>The version number information is used for changes in the format of External File
 		 List Message and is described here:
		 <table class=list>
		     <tr>
			<th width="30%">Version</th>
			<th align=left>Description</th>
		     </tr>
		     <tr>
			<td align=center><code>0</code></td>
			<td>Never used.
		     </tr>
		     <tr>
			<td align=center><code>1</code></td>
			<td>The current version used by the library.
		     </tr>
		  </table>
	      </P>
	    </td>
	  </tr>

	  <tr>
	    <td>Reserved</td>
	    <td>
	      <P>This field is reserved for future use.</P>
	    </td>
	  </tr>

	  <tr>
	    <td>Allocated Slots</td>
	    <td>
	      <P>The total number of slots allocated in the message.  Its value must be at least as
		 large as the value contained in the Used Slots field. (The current library simply
		 uses the number of Used Slots for this message)</P>
	    </td>
	  </tr>

	  <tr>
	    <td>Used Slots</td>
	    <td>
	      <P>The number of initial slots which contains valid information.</P>
	    </td>
	  </tr>

	  <tr>
	    <td>Heap Address</td>
	    <td>
	      <P>This is the address of a local heap which contains the names for the external
		 files (The local heap information can be found in Disk Format Level 1D in this
		 document).  The name at offset zero in the heap is always the empty string.</P>
	    </td>
	  </tr>

	  <tr>
	    <td>Slot Definitions</td>
	    <td>
	      <P>The slot definitions are stored in order according to the array addresses they
		 represent.</P>
	    </td>
	  </tr>

	</table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  External File List Slot
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>Name Offset(&lt;size&gt; bytes)<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>File Offset(&lt;size&gt; bytes)<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Size<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
	<table class=desc>
	  <tr>
	    <th width="30%">Field Name</th>
	    <th>Description</th>
	  </tr>

	  <tr>
	    <td>Name Offset(&lt;size&gt; bytes)</td>
	    <td>
		<P>The byte offset within the local name heap for the name
	    	of the file.  File names are stored as a URL which has a
	    	protocol name, a host name, a port number, and a file
	    	name:
		<code><em>protocol</em>:<em>port</em>//<em>host</em>/<em>file</em></code>.
		If the protocol is omitted then "file:" is assumed.  If
		the port number is omitted then a default port for that
		protocol is used.  If both the protocol and the port
		number are omitted then the colon can also be omitted. If
		the double slash and host name are omitted then
		"localhost" is assumed.  The file name is the only
		mandatory part, and if the leading slash is missing then
		it is relative to the application's current working
		directory (the use of relative names is not
		recommended).</P>
	    </td>
	  </tr>

	  <tr>
	    <td>File Offset(&lt;size&gt; bytes)</td>
	    <td>
	  	<P>This is the byte offset to the start of the data in the
	        specified file. For files that contain data for a single
	        dataset this will usually be zero.</P>
	    </td>
	  </tr>

	  <tr>
	    <td>Size</td>
	    <td>
		<P>This is the total number of bytes reserved in the
		specified file for raw data storage.  For a file that
		contains exactly one complete dataset which is not
		extendable, the size will usually be the exact size of the
		dataset.  However, by making the size larger one allows
		HDF5 to extend the dataset. The size can be set to a value
		larger than the entire file since HDF5 will read zeros
		past the end of the file without failing.</P>
	    </td>
	  </tr>
	</table>
    </div>


    <hr>
    <h4><a name="LayoutMessage">Name: Data Storage - Layout</a></h4>

    <P class=item><B>Header Message Type:</B> 0x0008</P>
    <P class=item><B>Length:</B> varies</P>
    <P class=item><B>Status:</B> Required for datasets, may not be repeated.</P>

    <P class=item><B>Purpose and Description:</B> Data layout describes how the
      elements of a multi-dimensional array are arranged in the linear
      address space of the file. Three types of data layout are
      supported:

    <ol>
      <li>Contiguous: The array can be stored in one contiguous area of the file.
	The layout requires that the size of the array be constant and
	does not permit chunking, compression, checksums, encryption,
	etc.  The message stores the total size of the array and the
	offset of an element from the beginning of the storage area is
	computed as in C.

      <li>Chunked: The array domain can be regularly decomposed into chunks and
	each chunk is allocated separately.  This layout supports
	arbitrary element traversals, compression, encryption, and
	checksums, and the chunks can be distributed across external
	raw data files (these features are described in other
	messages).  The message stores the size of a chunk instead of
	the size of the entire array; the size of the entire array can
	be calculated by traversing the B-tree that stores the chunk
	addresses.

      <li>Compact: The array can be stored in one contiguous block, as part of
        this object header message (this is called "compact" storage below).
    </ol>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Data Layout Message (Versions 1 and 2)
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Dimensionality</td>
	  <td>Layout Class</td>
	  <td>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4><br>Address<br><br></td>
	</tr>

	<tr>
	  <td colspan=4>Dimension 0 (4-bytes)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension 1 (4-bytes)</td>
	</tr>

	<tr>
	  <td colspan=4>...</td>
	</tr>

	<tr>
	  <td colspan=4>Dataset Element Size <em>(optional)</em></td>
	</tr>

	<tr>
	  <td colspan=4>Compact Data Size (4-bytes)</td>
	</tr>

	<tr>
	  <td colspan=4><br>Compact Data...<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
	  <th width="30%">Field Name</th>
	  <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>
	    <P>The version number information is used for changes in the format of the data
	    layout message and is described here:</P>
	      <table class=list>
		<tr>
		  <th width="30%">Version</th>
		  <th align=left>Description</th>
		</tr>

		<tr>
		  <td align=center><code>0</code></td>
		  <td>Never used.</td>
		</tr>

		<tr>
		  <td align=center><code>1</code></td>
		  <td>Used by version 1.4 and before of the library to encode layout information.
		      Data space is always allocated when the data set is created.</td>
		</tr>

		<tr>
		  <td align=center><code>2</code></td>
		  <td>Used by version 1.6.x of the library to encode layout information.
		      Data space is allocated only when it is necessary.</td>
		</tr>
	      </table>
	  </td>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td><P>An array has a fixed dimensionality.  This field
	    specifies the number of dimension size fields later in the
	    message.</P></td>
	</tr>

	<tr>
	  <td>Layout Class</td>
	  <td><P>The layout class specifies how the other fields of the
	    layout message are to be interpreted.  A value of one
	    indicates contiguous storage, a value of two indicates chunked storage,
	    while a value of zero indicates compact storage.  Other values will be defined
	    in the future.</P></td>
	</tr>

	<tr>
	  <td>Address</td>
	  <td><P>For contiguous storage, this is the address of the first
	    byte of storage.  For chunked storage this is the address
	    of the B-tree that is used to look up the addresses of the
	    chunks.  This field is not present for compact storage.
            If the version for this message is set to 2, the address
            may have the "undefined address" value, to indicate that
            storage has not yet been allocated for this array.</P></td>
	</tr>

	<tr>
	  <td>Dimensions</td>
	  <td><P>For contiguous and compact storage the dimensions define
            the entire size of the array while for chunked storage they define
            the size of a single chunk.  In all cases, they are in units of
            array elements (not bytes).  The first dimension stored in the list
            of dimensions is the slowest changing dimension and the last
            dimension stored is the fastest changing dimension.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dataset Element Size</td>
	  <td><P>The size of a dataset element, in bytes.  This field is only
            present for chunked storage.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Compact Data Size</td>
	  <td><P>This field is only present for compact data storage.
            It contains the size of the raw data for the dataset array.</P></td>

	<tr>
	  <td>Compact Data</td>
	  <td><P>This field is only present for compact data storage.
            It contains the raw data for the dataset array.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <P>Version 3 of this message re-structured the format into specific
        properties that are required for each layout class.

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  <B>Data Layout Message (Version 3)</B>
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Layout Class</td>
	  <td colspan=2 bgcolor=#DDDDDD>&nbsp;</td>
	</tr>

	<tr>
	  <td colspan=4><br>Properties<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
	  <th width="30%">Field Name</th>
	  <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>
	    <P>The version number information is used for changes in the format of layout message
	       and is described here:</P>
	      <table class=list>
		<tr>
		  <th width="30%">Version</th>
		  <th align=left>Description</th>
		</tr>

		<tr>
		  <td align=center><code>3</code></td>
		  <td>Used by the version 1.6.3 and later of the library to store properties
		      for each layout class.</td>
		</tr>
	      </table>
	  </td>
	</tr>

	<tr>
	  <td>Layout Class</td>
	  <td><P>The layout class specifies how the other fields of the layout message are to be
	      interpreted.  A value of one indicates contiguous storage, a value of two
	      indicates chunked storage, while a value of zero indicates compact storage.</P></td>
	</tr>

	<tr>
	  <td>Properties</td>
	  <td><P>This variable-sized field encodes information specific to each
            layout class and is described below.  If there is no property
            information specified for a layout class, the size of this field
            is zero bytes.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <P>Class-specific information for compact layout (Class 0):  (Note: The dimensionality information
       is in the Dataspace message)

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Descriptions
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=2>Size</td>
	  <td colspan=2 bgcolor=#DDDDDD>&nbsp;</td>
	</tr>

	<tr>
	  <td colspan=4><br>Raw Data...<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Size</td>
	  <td><P>This field contains the size of the raw data for the dataset array.</P></td>
	</tr>

	<tr>
	  <td>Raw Data</td>
	  <td><P>This field contains the raw data for the dataset array.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <P>Class-specific information for contiguous layout (Class 1):  (Note: The dimensionality information
       is in the Dataspace message)

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Descriptions
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>Address<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Size<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Address</td>
	  <td><P>This is the address of the first byte of raw data storage.
            The address may have the "undefined address" value, to indicate
            that storage has not yet been allocated for this array.</P></td>
	</tr>

	<tr>
	  <td>Size</td>
	  <td><P>This field contains the size allocated to store the raw data.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <P>Class-specific information for chunked layout (Class 2):

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Property Descriptions
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td colspan=3 bgcolor=#DDDDDD>&nbsp;</td>
	</tr>

	<tr>
	  <td colspan=4><br>Address<br><br></td>
	</tr>

	<tr>
	  <td colspan=4>Dimension 0 (4-bytes)</td>
	</tr>

	<tr>
	  <td colspan=4>Dimension 1 (4-bytes)</td>
	</tr>

	<tr>
	  <td colspan=4>...</td>
	</tr>

	<tr>
	  <td colspan=4>Dataset Element Size</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Dimensionality</td>
	  <td><P>A chunk has a fixed dimensionality.  This field specifies
	    the number of dimension size fields later in the message.</P></td>
	</tr>

	<tr>
	  <td>Address</td>
	  <td><P>This is the address of the B-tree that is used to look up the addresses of the
	    chunks.  The address may have the "undefined address" value, to indicate that
            storage has not yet been allocated for this array.</P></td>
	</tr>

	<tr>
	  <td>Dimensions</td>
	  <td><P>These values define the dimension size of a single chunk, in
            units of array elements (not bytes).  The first dimension stored in
            the list of dimensions is the slowest changing dimension and the
            last dimension stored is the fastest changing dimension.
            </P>
          </td>
	</tr>

	<tr>
	  <td>Dataset Element Size</td>
	  <td><P>The size of a dataset element, in bytes.
            </P>
          </td>
	</tr>
      </table>
    </div>

    <hr>
    <h4><a name="ReservedMessage_0009">Name: Reserved - Not Assigned Yet</a></h4>
    <P class=item><B>Header Message Type:</B> 0x0009</P>
    <P class=item><B>Length:</B> N/A</P>
    <P class=item><B>Status:</B> N/A</P>
    <P class=item><B>Format of Data:</B> N/A</P>

    <P class=item><B>Purpose and Description:</B> This message type was skipped during the initial
	specification of the file format and may be used in a future expansion to the format.

    <hr>
    <h4><a name="ReservedMessage_000A">Name: Reserved - Not Assigned Yet</a></h4>
    <P class=item><B>Header Message Type:</B> 0x0009</P>
    <P class=item><B>Length:</B> N/A</P>
    <P class=item><B>Status:</B> N/A</P>
    <P class=item><B>Format of Data:</B> N/A</P>

    <P class=item><B>Purpose and Description:</B> This message type was skipped during the initial
	specification of the file format and may be used in a future expansion to the format.

    <hr>
    <h4><a name="FilterMessage">Name: Data Storage - Filter Pipeline</a></h4>
    <P class=item><B>Header Message Type:</B> 0x000B</P>
    <P class=item><B>Length:</B> varies</P>
    <P class=item><B>Status:</B> Optional, may not be repeated.</P>

    <P class=item><B>Description:</B> This message describes the
      filter pipeline which should be applied to the data stream by
      providing filter identification numbers, flags, a name, and
      client data.</P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Filter Pipeline Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Number of Filters</td>
	  <td colspan=2>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4><br>Filter List<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td><P>The version number for this message.  This document
	    describes version 1.</P></td>
	</tr>

	<tr>
	  <td>Number of Filters</td>
	  <td><P>The total number of filters described by this
	    message. The maximum possible number of filters in a
	    message is 32.</P></td>
	</tr>

	<tr>
	  <td>Filter List</td>
	  <td><P>A description of each filter.  A filter description
	    appears in the next table.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Filter Description
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=2>Filter Identification</td>
	  <td colspan=2>Name Length</td>
	</tr>

	<tr>
	  <td colspan=2>Flags</td>
	  <td colspan=2>Number of Values for Client Data</td>
	</tr>

	<tr>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Client Data<br><br></td>
	</tr>

	<tr>
	  <td colspan=4>Padding</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Filter Identification</td>
	  <td>
            <p>
            This value, often referred to as a filter identifier,
            is designed to be a unique identifier for the filter.
            Values from zero through 32,767 are reserved for filters
            supported by The HDF Group in the HDF5 library and for
            filters requested and supported by third parties.
            Filters supported by The HDF Group are documented immediately
            below.  Information on 3rd-party filters can be found at
            <a href="/services/contributions.html#filters">
            <code>https://support.hdfgroup.org/services/contributions.html#filters</code></a>.
            <a href="#Footnote1Change"><sup><small>1</small></sup></a>
            <p>
            To request a filter identifier, please contact
            The HDF Group&rsquo;s Help Desk at
            <img src="Graphics/help.png" valign="center" height=14>.
            You will be asked to provide the following information:
            <ol>
              <li>Contact information for the developer requesting the
                  new identifier
              <li>A short description of the new filter
              <li>Links to any relevant information, including licensing
                  information
            </ol>
            <p>
            Values from 32768 to 65535 are reserved for non-distributed uses
            (for example, internal company usage) or for application usage
            when testing a feature. The HDF Group does not track or document
            the use of the filters with identifiers from this range.

            <p>
            The filters currently in library version 1.6.5 are
	    listed below:
	    <table class=list>
	      <tr>
		<th width="30%">Identification</th>
		<th align=left>Name</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>1</code></td>
		<td>deflate</td>
		<td>GZIP deflate compression</td>
	      </tr>

	      <tr>
		<td align=center><code>2</code></td>
		<td>shuffle</td>
		<td>Data element shuffling</td>
	      </tr>

	      <tr>
		<td align=center><code>3</code></td>
		<td>fletcher32</td>
		<td>Fletcher32 checksum</td>
	      </tr>

	      <tr>
		<td align=center><code>4</code></td>
		<td>szip</td>
		<td>SZIP compression</td>
	      </tr>
	    </table>
	  </P></td>
	</tr>

	<tr>
	  <td>Name Length</td>
	  <td><P>Each filter has an optional null-terminated ASCII name
	    and this field holds the length of the name including the
	    null termination padded with nulls to be a multiple of
	    eight. If the filter has no name then a value of zero is
	    stored in this field.</P></td>
	</tr>

	<tr>
	  <td>Flags</td>
	  <td><P>The flags indicate certain properties for a filter.  The
	    bit values defined so far are:</P>
	    <table class=list>
	      <tr>
		<th width="30%">Value</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>bit 1</code></td>
	        <td>If set then the filter is an optional filter.
		During output, if an optional filter fails it will be
		silently removed from the pipeline.</td>
	      </tr>
	    </table>
	  </td>
	</tr>

	<tr>
	  <td>Client Data Number of Values</td>
	  <td><P>Each filter can store a few integer values to control
	    how the filter operates.  The number of entries in the
	    Client Data array is stored in this field.</P></td>
	</tr>

	<tr>
	  <td>Name</td>
	  <td><P>If the Name Length field is non-zero then it will
	    contain the size of this field, a multiple of eight.  This
	    field contains a null-terminated, ASCII character
	    string to serve as a comment/name for the filter.</P></td>
	</tr>

	<tr>
	  <td>Client Data</td>
	  <td><P>This is an array of four-byte integers which will be
	    passed to the filter function.  The Client Data Number of
	    Values determines the number of elements in the array.</P></td>
	</tr>

	<tr>
	  <td>Padding</td>
	  <td><P>Four bytes of zeros are added to the message at this
	    point if the Client Data Number of Values field contains
	    an odd number.</P></td>
	</tr>
      </table>
    </div>
    <p>
    <hr align="left" width="50">
    <a name="Footnote1Change"><sup>1</sup></a>If you are reading
    an earlier version of this document, this link may have changed.
    If the link does not work, use the latest version of this document
    on <a href="https://support.hdfgroup.org">The HDF Group</a>&rsquo;s website,
    <a href="/HDF5/doc/H5.format.html">
    <code>https://support.hdfgroup.org/HDF5/doc/H5.format.html</code></a>;
    the link there will always be correct.
    <small><a href="#FilterMessage">(Return)</a>
    </P>

    <hr>
    <h4><a name="AttributeMessage">Name: Attribute</a></h4>
    <P class=item><B>Header Message Type:</B> 0x000C
    <P class=item><B>Length:</B> varies
    <P class=item><B>Status:</B> Optional, may be repeated.

    <P class=item><B>Description:</B>  The <em>Attribute</em>
      message is used to list objects in the HDF file which are used
      as attributes, or "metadata" about the current object.  An
      attribute is a small dataset; it has a name, a datatype, a data
      space, and raw data.  Since attributes are stored in the object
      header they must be relatively small (<64KB) and can be
      associated with any type of object which has an object header
      (groups, datasets, named types and spaces, etc.).

    <P class=item2>Note: Attributes on an object must have unique names.  (The HDF5 library
      currently enforces this by causing the creation of an attribute with
      a duplicate name to fail).  Attributes on different objects may have the
      same name, however.

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Attribute Message (Version 1)
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Reserved</td>
	  <td colspan=2>Name Size</td>
	</tr>

	<tr>
	  <td colspan=2>Datatype Size</td>
	  <td colspan=2>Dataspace Size</td>
	</tr>

	<tr>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Datatype<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Dataspace<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Data<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td><P>The version number information is used for changes in the format of the
	    attribute message and is described here:</P>
	    <table class=list>
	      <tr>
		<th width="30%">Version</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>0</code></td>
		<td>Never used.</td>
	      </tr>

	      <tr>
		<td align=center><code>1</code></td>
		<td>Used by the library before version 1.6 to encode attribute message.
		    This version does not support shared data type.</td>
	      </tr>
	    </table>
	  </td>
	</tr>

	<tr>
	  <td>Reserved</td>
	  <td><P>This field is reserved for later use and is set to
	    zero.</P></td>
	</tr>

	<tr>
	  <td>Name Size</td>
	  <td><P>The length of the attribute name in bytes including the
	    null terminator.  Note that the Name field below may
	    contain additional padding not represented by this
	    field.</P></td>
	</tr>

	<tr>
	  <td>Datatype Size</td>
	  <td><P>The length of the datatype description in the Datatype
	    field below.  Note that the Datatype field may contain
	    additional padding not represented by this field.</P></td>
	</tr>

	<tr>
	  <td>Dataspace Size</td>
	  <td><P>The length of the dataspace description in the Dataspace
	    field below.  Note that the Dataspace field may contain
	    additional padding not represented by this field.</P></td>
	</tr>

	<tr>
	  <td>Name</td>
	  <td><P>The null-terminated attribute name.  This field is
	    padded with additional null characters to make it a
	    multiple of eight bytes.</P></td>
	</tr>

	<tr>
	  <td>Datatype</td>
	  <td><P>The datatype description follows the same format as
	    described for the datatype object header message.  This
	    field is padded with additional zero bytes to make it a
	    multiple of eight bytes.</P></td>
	</tr>

	<tr>
	  <td>Dataspace</td>
	  <td><P>The dataspace description follows the same format as
	    described for the dataspace object header message.  This
	    field is padded with additional zero bytes to make it a
	    multiple of eight bytes.</P></td>
	</tr>

	<tr>
	  <td>Data</td>
	  <td><P>The raw data for the attribute.  The size is determined
	    from the datatype and dataspace descriptions.  This
	    field is <em>not</em> padded with additional bytes.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Attribute Message (Version 2)
	</caption>

	<tr align=center>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Flag</td>
	  <td colspan=2>Name Size</td>
	</tr>

	<tr>
	  <td colspan=2>Type Size</td>
	  <td colspan=2>Space Size</td>
	</tr>

	<tr>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Type<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Space<br><br></td>
	</tr>

	<tr>
	  <td colspan=4><br>Data<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td><P>The version number information is used for changes in the format of the
	    attribute message and is described here:</P>
	    <table class=list width="90%">
	      <tr>
		<th width="30%">Version</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>2</code></td>
		<td>Used by the library of version 1.6.x and after to encode attribute message.
		    This version supports shared data type.  The fields of name, type, and space
		    are not padded with additional bytes of zero.</td>
	      </tr>
	    </table>
	  </td>
	</tr>

	<tr>
	  <td>Flag</td>
	  <td><P>This field indicates whether the data type of this attribute is shared:</P>
	    <table class=list width="90%">
	      <tr>
		<th width="30%">Value</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>0</code></td>
		<td>Datatype is <em>not</em> shared.</td>
	      </tr>

	      <tr>
		<td align=center><code>1</code></td>
		<td>Datatype is shared.</td>
	      </tr>
	    </table>
	  </td>
	</tr>

	<tr>
	  <td>Name Size</td>
	  <td><P>The length of the attribute name in bytes including the
	    null terminator.</P></td>
	</tr>

	<tr>
	  <td>Datatype Size</td>
	  <td><P>The length of the datatype description in the Datatype
	    field below.</P></td>
	</tr>

	<tr>
	  <td>Dataspace Size</td>
	  <td><P>The length of the dataspace description in the Dataspace
	    field below.</P></td>
	</tr>

	<tr>
	  <td>Name</td>
	  <td><P>The null-terminated attribute name.  This field is <em>not</em>
	    padded with additional bytes.</P></td>
	</tr>

	<tr>
	  <td>Datatype</td>
	  <td><P>The datatype description follows the same format as
	    described for the datatype object header message.  This
	    field is <em>not</em> padded with additional bytes.</P></td>
	</tr>

	<tr>
	  <td>Dataspace</td>
	  <td><P>The dataspace description follows the same format as
	    described for the dataspace object header message.  This
	    field is <em>not</em> padded with additional bytes.</P></td>
	</tr>

	<tr>
	  <td>Data</td>
	  <td><P>The raw data for the attribute.  The size is determined
	    from the datatype and dataspace descriptions.  This
	    field is <em>not</em> padded with additional zero
	    bytes.</P></td>
	</tr>
      </table>
    </div>

    <hr>
    <h4><a name="CommentMessage">Name: Object Comment</a></h4>

    <P class=item><B>Header Message Type:</B> 0x000D</P>
    <P class=item><B>Length:</B> varies</P>
    <P class=item><B>Status:</B> Optional, may not be repeated.</P>

    <P class=item><B>Description:</B> The object comment is
      designed to be a short description of an object.  An object comment
      is a sequence of non-zero (<code>\0</code>) ASCII characters with no other
      formatting included by the library.</P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Name Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=4><br>Comment<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Name</td>
	  <td>A null terminated ASCII character string.</td>
	</tr>
      </table>
    </div>

    <hr>
    <h4><a name="OldModifiedMessage">Name: Object Modification Date &amp; Time (Old)</a></h4>

    <P class=item><B>Header Message Type:</B> 0x000E</P>
    <P class=item><B>Length:</B> fixed</P>
    <P class=item><B>Status:</B> Optional, may not be repeated.</P>

    <P class=item><B>Description:</B>  The object modification date
      and time is a timestamp which indicates (using ISO-8601 date and
      time format) the last modification of an object.  The time is
      updated when any object header message changes according to the
      system clock where the change was posted.

      <br><br>This modification time message is deprecated in favor of the "new"
      modification time message (Message Type 0x0012) and is no longer written
      to the file in versions of the HDF5 library after the 1.6.0 version.
    </P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Modification Time Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td colspan=4>Year</td>
	</tr>

	<tr>
	  <td colspan=2>Month</td>
	  <td colspan=2>Day of Month</td>
	</tr>

	<tr>
	  <td colspan=2>Hour</td>
	  <td colspan=2>Minute</td>
	</tr>

	<tr>
	  <td colspan=2>Second</td>
	  <td colspan=2>Reserved</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Year</td>
	  <td><P>The four-digit year as an ASCII string. For example,
	    <code>1998</code>.  All fields of this message should be interpreted
	    as coordinated universal time (UTC)</P></td>
	</tr>

	<tr>
	  <td>Month</td>
	  <td><P>The month number as a two digit ASCII string where
	    January is <code>01</code> and December is <code>12</code>.</P></td>
	</tr>

	<tr>
	  <td>Day of Month</td>
	  <td><P>The day number within the month as a two digit ASCII
	    string. The first day of the month is <code>01</code>.</P></td>
	</tr>

	<tr>
	  <td>Hour</td>
	  <td><P>The hour of the day as a two digit ASCII string where
	    midnight is <code>00</code> and 11:00pm is <code>23</code>.</P></td>
	</tr>

	<tr>
	  <td>Minute</td>
	  <td><P>The minute of the hour as a two digit ASCII string where
	    the first minute of the hour is <code>00</code> and
	    the last is <code>59</code>.</P></td>
	</tr>

	<tr>
	  <td>Second</td>
	  <td><P>The second of the minute as a two digit ASCII string
	    where the first second of the minute is <code>00</code>
	    and the last is <code>59</code>.</P></td>
	</tr>

	<tr>
	  <td>Reserved</td>
	  <td><P>This field is reserved and should always be zero.</P></td>
	</tr>
      </table>
    </div>

    <hr>
    <h4><a name="SharedMessage">Name: Shared Object Message</a></h4>
    <P class=item><B>Header Message Type:</B> 0x000F</P>
    <P class=item><B>Length:</B> Fixed</P>
    <P class=item><B>Status:</B> Optional, may be repeated.</P>

    <P class=item><B>Description:</B>  A constant message can be shared among
      several object headers.  A <em>Shared Object</em> Message contains the address of
      the object message to be shared.  Care must be exercised to prevent cycles when a
      message of one object header points to a message in some other object header.
      Starting from Version 2 of the Shared Object Message, the <em>Flags</em>
      field becomes unused.
    </P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Shared Object Message (Version 1)
	</caption>

	<tr>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Flags</td>
	  <td colspan=2>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4><br>Pointer<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td><P>The version number is used when there are changes in the format
            of a shared object message and is described here:</P>
	    <table class=list>
	      <tr>
		<th width="30%">Version</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>0</code></td>
		<td>Never used.</td>
	      </tr>

	      <tr>
		<td align=center><code>1</code></td>
		<td>Used by the library before version 1.6.1.  In this version,
		  the Flags field is used to indicate whether the actual message is
		  stored in the global heap (never implemented).  The Pointer field
		  either contains the header message address in the global heap
		  (never implemented) or the address of the shared object header.</td>
	      </tr>
	    </table>
	</tr>

	<tr>
	  <td>Flags</td>
	  <td><P>The Shared Message message points to a message which is
	    shared among multiple object headers.  The Flags field
	    describes the type of sharing:</P>
	    <table class=list>
	      <tr>
		<th width="30%">Bit</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>0</code></td>
		<td>If this bit is clear then the actual message is the
                  first message in some other object header; otherwise
                  the actual message is stored in the global heap (never
		  implemented).</td>
	      </tr>

	      <tr>
		<td align=center><code>2-7</code></td>
		<td>Reserved (always zero)</td>
	      </tr>
	    </table>
	  </td>
	</tr>

	<tr>
	  <td>Pointer</td>
	  <td><P>The address of the object header
            containing the message to be shared.</P></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=format>
	<caption>
	  Shared Object Message (Version 2)
	</caption>

	<tr>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	</tr>

	<tr>
	  <td>Version</td>
	  <td>Flags</td>
	  <td colspan=2 bgcolor=#DDDDDD>&nbsp;</td>
	</tr>

	<tr>
	  <td colspan=4><br>Pointer<br><br></td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td><P>The version number is used when there are changes in the format
            of a shared object message and is described here:</P>
	    <table class=list>
	      <tr>
		<th width="30%">Version</th>
		<th align=left>Description</th>
	      </tr>

	      <tr>
		<td align=center><code>2</code></td>
		<td>Used by the library of version 1.6.1 and after.  In this version,
		  The Flags field is not used and the Pointer field contains the address
		  of the object header containing the message to be shared. </td>
	      </tr>
	    </table>
	</tr>

	<tr>
	  <td>Flags</td>
	  <td><P>Unused.</P></td>
	</tr>

	<tr>
	  <td>Pointer</td>
	  <td><P>The address of the object header
            containing the message to be shared.</P></td>
	</tr>
      </table>
    </div>


    <hr>
    <h4><a name="ContinuationMessage">Name: Object Header Continuation</a></h4>
    <P class=item><B>Header Message Type:</B> 0x0010</P>
    <P class=item><B>Length:</B> fixed</P>
    <P class=item><B>Status:</B> Optional, may be repeated.</P>
    <P class=item><B>Description:</B>  The object header continuation is the location
        in the file of more header messages for the current data object.  This can be
        used when header blocks become too large or are likely to change over time.</P>

    <P class=item><B>Format of Data:</B>
    <br>
    <div align=center>
      <table class=format>
        <caption>
          Object Header Continuation Message
        </caption>

        <tr>
          <th width=25%>byte</th>
          <th width=25%>byte</th>
          <th width=25%>byte</th>
          <th width=25%>byte</th>
        </tr>

        <tr>
          <td colspan=4><br>Offset<br><br></td>
        </tr>

        <tr>
          <td colspan=4><br>Length<br><br></td>
        </tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
        <tr>
          <th width=30%>Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>Offset</td>
          <td><P>This value is the offset in bytes from the beginning of the file where the
              header continuation information is located.</P></td>
        </tr>

        <tr>
          <td>Length</td>
          <td><P>This value is the length in bytes of the header continuation information in
              the file.</P></td>
        </tr>
      </table>
    </div>

    <hr>
    <h4><a name="SymbolTableMessage">Name: Group Message</a></h4>
    <P class=item><B>Header Message Type:</B> 0x0011</P>
    <P class=item><B>Length:</B> fixed</P>
    <P class=item><B>Status:</B> Required for groups, may not be repeated.</P>
    <P class=item><B>Description:</B> Each group has a B-tree and a
        name heap which are pointed to by this message.</P>
    <P class=item><B>Format of data:</B>

    <br>
    <div align=center>
      <table class=format>
        <caption>
          <B>Group Message</B>
        </caption>

        <tr>
          <th width="25%">byte</th>
          <th width="25%">byte</th>
          <th width="25%">byte</th>
          <th width="25%">byte</th>
        </tr>

        <tr>
          <td colspan=4><br>B-tree Address<br><br></td>
        </tr>

        <tr>
          <td colspan=4><br>Heap Address<br><br></td>
        </tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
        <tr>
          <th width=30%>Field Name</th>
          <th>Description</th>
        </tr>

        <tr>
          <td>B-tree Address</td>
          <td><P>This value is the offset in bytes from the beginning of the file
            where the B-tree is located.</P></td>
        </tr>

        <tr>
          <td>Heap Address</td>
          <td><P>This value is the offset in bytes from the beginning of the file
            where the group name heap is located.</P></td>
        </tr>
      </table>
    </div>

    <hr>
    <h4><a name="ModifiedMessage">Name: Object Modification Date &amp; Time</a></h4>

    <P class=item><B>Header Message Type:</B> 0x0012 </P>
    <P class=item><B>Length:</B> Fixed </P>
    <P class=item><B>Status:</B> Optional, may not be repeated. </P>

    <P class=item><B>Description:</B> The object modification date
      and time is a timestamp which indicates the last modification of an object.
      The time is updated when any object header message changes according to the
      system clock where the change was posted.
    </P>

    <P class=item><B>Format of Data:</B>
    <div align=center>
      <table class=format>
	<caption>
	  Modification Time Message
	</caption>

	<tr>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	  <th width="25%">byte</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td colspan=3>Reserved</td>
	</tr>

	<tr>
	  <td colspan=4>Seconds After Epoch</td>
	</tr>
      </table>
    </div>

    <br>
    <div align=center>
      <table class=desc>
	<tr>
          <th width="30%">Field Name</th>
          <th>Description</th>
	</tr>

	<tr>
	  <td>Version</td>
	  <td><P>The version number is used for changes in the format of Object Modification Time
	      and is described here:</P>
	     <table class=list>
		<tr>
		  <th width="30%">Version</th>
		  <th align=left>Description</th>
		</tr>

		<tr>
		  <td align=center><code>0</code></td>
		  <td>Never used.</td>
		</tr>

		<tr>
		  <td align=center><code>1</code></td>
		  <td>Used by Version 1.6.1 and after of the library to encode time.  In
		    this version, the time is the seconds after Epoch.</td>
		</tr>
	      </table>
	  </td>
	</tr>

	<tr>
	  <td>Reserved</td>
	  <td><P>This field is reserved and should always be zero.</P></td>
	</tr>

	<tr>
	  <td>Seconds After Epoch</td>
	  <td><P>The number of seconds since 0 hours, 0 minutes, 0 seconds,
	    January 1, 1970, Coordinated Universal Time.</P></td>
	</tr>
      </table>
    </div>

<hr>
<h3><a name="DataStorage">Disk Format: Level 2b - Data Object Data Storage</a></h3>
<P>The data for an object is stored separately from the header
information in the file and may not actually be located in the HDF5 file
itself if the header indicates that the data is stored externally.  The
information for each record in the object is stored according to the
dimensionality of the object (indicated in the dimensionality header message).
Multi-dimensional data is stored in C order [same as current scheme], i.e. the
"last" dimension changes fastest.
<P>Data whose elements are composed of simple number-types are stored in
native-endian IEEE format, unless they are specifically defined as being stored
in a different machine format with the architecture-type information from the
number-type header message.  This means that each architecture will need to
[potentially] byte-swap data values into the internal representation for that
particular machine.
<P> Data with a variable-length datatype is stored in the global heap
of the HDF5 file.  Global heap identifiers are stored in the
data object storage.
<P>Data whose elements are composed of pointer number-types are stored in several
different ways depending on the particular pointer type involved. Simple
pointers are just stored as the dataset offset of the object being pointed to with the
size of the pointer being the same number of bytes as offsets in the file.
Dataset region references are stored as a heap-ID which points to the following
information within the file-heap: an offset of the object pointed to, number-type
information (same format as header message), dimensionality information (same
format as header message), sub-set start and end information (i.e. a coordinate
location for each), and field start and end names (i.e.  a [pointer to the]
string indicating the first field included and a [pointer to the] string name
for the last field).

<P>Data of a compound datatype is stored as a contiguous stream of the items
in the structure, with each item formatted according to its datatype.</p>

<hr>
<h3><a name="Appendix">Appendix</a></h3>
<P>Definitions of various terms used in this document.
</P>
<P>The <A name="UndefinedAddress">"undefined address"</A> for a file is a
file address with all bits set, i.e. <code>0xffff...ff</code>.
<P>The <A name="UnlimitedDim">"unlimited size"</A> for a size is a
value with all bits set, i.e. <code>0xffff...ff</code>.

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